[WIP][confmap] Enable decoding nil values in confmap #24925

Job	Run time
govulncheck	39s
setup-environment	12s
lint	4m 37s
checks	2m 10s
test-coverage	8m 55s
cross-build-collector (darwin, amd64)	46s
unittest-matrix (ubuntu-latest, ~1.23)	2m 23s
cross-build-collector (darwin, arm64)	48s
cross-build-collector (linux, 386)	43s
cross-build-collector (linux, amd64)	50s
unittest-matrix (ubuntu-latest, ~1.22)	2m 21s
cross-build-collector (linux, arm64)	44s
cross-build-collector (linux, ppc64le)	40s
cross-build-collector (linux, arm, 7)	57s
cross-build-collector (linux, s390x)	39s
cross-build-collector (windows, 386)	51s
cross-build-collector (windows, amd64)	41s
unittest	1s
	28m 57s

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