Fix null handling for enum types.

[bzbarsky-apple]

For enum types we ended up with an incorrect null representation via
NumericAttributeTraits, because while we called GetNullValue with the
template argument set to our underlying type the actual implementation
ignores the template argument: it's just there for the enable_if
tests.

As far as testing goes, the yaml tests being added would have caught
this if our attribute store interaction were actually using enums, but
that code switches to treating them as just bare ints very early,
before instantiating the NumericAttributeTraits template. The unit
test does catch this problem, though.

Problem

See above.

Change overview

See above.

Testing

See above.

[github-actions]

PR #13091: Size comparison from 8e40abc to f98d2e6

Increases above 0.2%:

platform	target	config	section	8e40abc	f98d2e6	change	% change
linux	chip-tool-ipv6only	arm64	.got	56160	56480	320	0.6
			.rodata	379476	380948	1472	0.4

Increases (1 build for linux)

platform	target	config	section	8e40abc	f98d2e6	change	% change
linux	chip-tool-ipv6only	arm64	(read only)	6930700	6946556	15856	0.2
			(read/write)	323633	323953	320	0.1
			.got	56160	56480	320	0.6
			.rodata	379476	380948	1472	0.4
			.text	5863652	5877076	13424	0.2

Full report (31 builds for efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	8e40abc	f98d2e6	change	% change
efr32	lighting-app	BRD4161A	(read only)	829456	829456	0	0.0
			(read/write)	127352	127352	0	0.0
			.bss	125472	125472	0	0.0
			.data	1876	1876	0	0.0
			.text	829448	829448	0	0.0
		BRD4161A+rpc	(read only)	817084	817084	0	0.0
			(read/write)	144016	144016	0	0.0
			.bss	142040	142040	0	0.0
			.data	1976	1976	0	0.0
			.text	817076	817076	0	0.0
	window-app	BRD4161A	(read only)	802848	802848	0	0.0
			(read/write)	126288	126288	0	0.0
			.bss	124456	124456	0	0.0
			.data	1832	1832	0	0.0
			.text	802840	802840	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	877072	877072	0	0.0
			(read/write)	1313042	1313042	0	0.0
			.dram0.bss	69784	69784	0	0.0
			.dram0.data	14220	14220	0	0.0
			.flash.rodata	175976	175976	0	0.0
			.flash.text	877072	877072	0	0.0
			.iram0.text	62254	62254	0	0.0
		m5stack	(read only)	937951	937951	0	0.0
			(read/write)	442144	442144	0	0.0
			.dram0.bss	74280	74280	0	0.0
			.dram0.data	34056	34056	0	0.0
			.flash.rodata	202800	202800	0	0.0
			.flash.text	932567	932567	0	0.0
			.iram0.text	122671	122671	0	0.0
k32w	light	k32w061+release	(read/write)	648312	648312	0	0.0
			.bss	76480	76480	0	0.0
			.data	1904	1904	0	0.0
			.text	564128	564128	0	0.0
	lock	k32w061+release	(read/write)	633028	633028	0	0.0
			.bss	76200	76200	0	0.0
			.data	1860	1860	0	0.0
			.text	549168	549168	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	6930700	6946556	15856	0.2
			(read/write)	323633	323953	320	0.1
			.bss	54577	54577	0	0.0
			.data	1096	1096	0	0.0
			.data.rel.ro	208096	208096	0	0.0
			.dynamic	560	560	0	0.0
			.got	56160	56480	320	0.6
			.init	24	24	0	0.0
			.init_array	168	168	0	0.0
			.rodata	379476	380948	1472	0.4
			.text	5863652	5877076	13424	0.2
	thermostat-no-ble	arm64	(read only)	1993780	1993780	0	0.0
			(read/write)	143937	143937	0	0.0
			.bss	64321	64321	0	0.0
			.data	880	880	0	0.0
			.data.rel.ro	72000	72000	0	0.0
			.dynamic	560	560	0	0.0
			.got	3840	3840	0	0.0
			.init	24	24	0	0.0
			.init_array	288	288	0	0.0
			.rodata	128196	128196	0	0.0
			.text	1654112	1654112	0	0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2334104	2334104	0	0.0
			.bss	189068	189068	0	0.0
			.data	5264	5264	0	0.0
			.text	1296680	1296680	0	0.0
	lighting-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2328616	2328616	0	0.0
			.bss	180896	180896	0	0.0
			.data	5544	5544	0	0.0
			.text	1291216	1291216	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2301712	2301712	0	0.0
			.bss	179944	179944	0	0.0
			.data	5536	5536	0	0.0
			.text	1264312	1264312	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	1140008	1140008	0	0.0
			.bss	11756	11756	0	0.0
			.data	4368	4368	0	0.0
			.text	103392	103392	0	0.0
	shell	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2053688	2053688	0	0.0
			.bss	156972	156972	0	0.0
			.data	4864	4864	0	0.0
			.text	1016288	1016288	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	(read/write)	935579	935579	0	0.0
			bss	118400	118400	0	0.0
			rodata	108120	108120	0	0.0
			text	631508	631508	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	921979	921979	0	0.0
			bss	115444	115444	0	0.0
			rodata	101536	101536	0	0.0
			text	626820	626820	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	859342	859342	0	0.0
			bss	116684	116684	0	0.0
			rodata	103044	103044	0	0.0
			text	558948	558948	0	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	907723	907723	0	0.0
			bss	117588	117588	0	0.0
			rodata	103424	103424	0	0.0
			text	609332	609332	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	831638	831638	0	0.0
			bss	115900	115900	0	0.0
			rodata	98388	98388	0	0.0
			text	536816	536816	0	0.0
	pigweed-app	nrf52840dk_nrf52840	(read/write)	542351	542351	0	0.0
			bss	52588	52588	0	0.0
			rodata	50668	50668	0	0.0
			text	376892	376892	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	912603	912603	0	0.0
			bss	117496	117496	0	0.0
			rodata	104768	104768	0	0.0
			text	612884	612884	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	905803	905803	0	0.0
			bss	117376	117376	0	0.0
			rodata	102896	102896	0	0.0
			text	608080	608080	0	0.0
	shell	nrf52840dk_nrf52840	(read/write)	796079	796079	0	0.0
			bss	109464	109464	0	0.0
			rodata	78096	78096	0	0.0
			text	532048	532048	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	708710	708710	0	0.0
			bss	107352	107352	0	0.0
			rodata	72396	72396	0	0.0
			text	449544	449544	0	0.0
p6	all-clusters-app	default	(read/write)	2384624	2384624	0	0.0
			.bss	117260	117260	0	0.0
			.data	2544	2544	0	0.0
			.text	1342888	1342888	0	0.0
	light-app	default	(read/write)	2324008	2324008	0	0.0
			.bss	106152	106152	0	0.0
			.data	2384	2384	0	0.0
			.text	1282272	1282272	0	0.0
	lock-app	default	(read/write)	2296216	2296216	0	0.0
			.bss	105032	105032	0	0.0
			.data	2336	2336	0	0.0
			.text	1254480	1254480	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	531812	531812	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	86816	86816	0	0.0
			.data	1004	1004	0	0.0
			.text	526492	526492	0	0.0
	lock-app	qpg6105+debug	(read only)	503492	503492	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	85952	85952	0	0.0
			.data	952	952	0	0.0
			.text	498172	498172	0	0.0
	persistent-storage-app	qpg6105+debug	(read only)	106448	106448	0	0.0
			(read/write)	146938	146938	0	0.0
			.bss	36146	36146	0	0.0
			.data	288	288	0	0.0
			.text	101128	101128	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	830230	830230	0	0.0
			bss	87040	87040	0	0.0
			noinit	37160	37160	0	0.0
			text	578418	578418	0	0.0