[styles] Only run the check on the client-side

[oliviertassinari]

A false positive was raised in #15745 (comment). I'm making the logic one step closer to styled-components.

🎁 and a couple of small changes.

[mui-pr-bot]

Details of bundle changes.

Comparing: 3de4acf...08983a0

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.01% 🔺	+0.02% 🔺	317,807	317,823	87,482	87,497
@material-ui/core/Paper	0.00%	-0.01%	68,279	68,279	20,368	20,366
@material-ui/core/Paper.esm	0.00%	+0.06% 🔺	61,574	61,574	19,151	19,162
@material-ui/core/Popper	0.00%	-0.06%	28,945	28,945	10,405	10,399
@material-ui/core/Textarea	0.00%	-0.08%	5,513	5,513	2,375	2,373
@material-ui/core/TrapFocus	0.00%	-0.06%	3,753	3,753	1,578	1,577
@material-ui/core/styles/createMuiTheme	+0.01% 🔺	+0.05% 🔺	16,008	16,009	5,790	5,793
@material-ui/core/useMediaQuery	0.00%	-0.09%	2,597	2,597	1,101	1,100
@material-ui/lab	0.00%	+0.01% 🔺	140,018	140,018	43,396	43,402
@material-ui/styles	0.00%	0.00%	51,699	51,699	15,346	15,346
@material-ui/system	0.00%	-0.02%	15,420	15,420	4,394	4,393
Button	0.00%	+0.08% 🔺	84,279	84,279	25,699	25,720
Modal	0.00%	+0.06% 🔺	14,107	14,107	5,100	5,103
Portal	+Infinity% 🔺	+Infinity% 🔺	0	3,473	0	1,572
Slider	0.00%	+0.06% 🔺	74,702	74,702	23,240	23,253
colorManipulator	0.00%	0.00%	3,904	3,904	1,544	1,544
docs.landing	0.00%	0.00%	55,119	55,119	13,940	13,940
docs.main	0.00%	0.00%	648,815	648,815	204,522	204,522
packages/material-ui/build/umd/material-ui.production.min.js	-0.00%	-0.01%	291,145	291,142	83,353	83,348

Generated by 🚫 dangerJS against 08983a0