[system] Fallback to value if theme's value is an array and index missing

[stasiukanya]

Fixes #16260

[mui-pr-bot]

Details of bundle changes.

Comparing: 9a538bc...1c899ef

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	334,815	334,818	91,340	91,342
@material-ui/core/Paper	0.00%	0.00%	69,248	69,248	20,556	20,556
@material-ui/core/Paper.esm	0.00%	0.00%	62,520	62,520	19,303	19,303
@material-ui/core/Popper	0.00%	0.00%	28,405	28,405	10,179	10,179
@material-ui/core/Textarea	0.00%	0.00%	5,106	5,106	2,149	2,149
@material-ui/core/TrapFocus	0.00%	0.00%	3,766	3,766	1,597	1,597
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,351	16,351	5,819	5,819
@material-ui/core/useMediaQuery	0.00%	0.00%	2,488	2,488	1,050	1,050
@material-ui/lab	0.00%	0.00%	143,407	143,407	43,822	43,822
@material-ui/styles	0.00%	0.00%	51,780	51,780	15,359	15,359
@material-ui/system	+0.02% 🔺	+0.05% 🔺	15,583	15,586	4,342	4,344
Button	0.00%	0.00%	79,690	79,690	24,259	24,259
Modal	0.00%	0.00%	14,542	14,542	5,114	5,114
Portal	0.00%	0.00%	2,907	2,907	1,316	1,316
Rating	0.00%	0.00%	70,240	70,240	21,944	21,944
Slider	0.00%	0.00%	75,381	75,381	23,270	23,270
colorManipulator	0.00%	0.00%	3,835	3,835	1,519	1,519
docs.landing	0.00%	0.00%	54,267	54,267	14,346	14,346
docs.main	0.00%	0.00%	599,265	599,268	191,022	191,022
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	305,706	305,709	87,931	87,933

Generated by 🚫 dangerJS against 1c899ef

[oliviertassinari]

@stasiukanya Thank you, do you think that you could look at adding a test?

[stasiukanya]

@oliviertassinari Yes, I've added test for shadows and moved it to separate file

[oliviertassinari]

Thanks for the extra test case! If you don't mind I have moved it with the other. I think that it's simpler to keep them all at the same place, at least, until the test file grows too much (+500 LOCs).

[oliviertassinari]

Thanks