[Modal] Fix autoFocus support

[oliviertassinari]

Fix #14545 (comment) @maniekins's feedback.

[mui-pr-bot]

Details of bundle changes.

Comparing: 6b68487...e9c1172

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.06% 🔺	+0.07% 🔺	368,621	368,835	91,396	91,462
@material-ui/core/Paper	0.00%	0.00%	76,647	76,647	19,297	19,297
@material-ui/core/Paper.esm	0.00%	0.00%	71,595	71,595	18,772	18,772
@material-ui/core/Popper	0.00%	0.00%	30,462	30,462	10,583	10,583
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,286	17,286	5,720	5,720
@material-ui/core/useMediaQuery	0.00%	0.00%	2,486	2,486	1,054	1,054
@material-ui/lab	0.00%	0.00%	182,889	182,889	50,246	50,246
@material-ui/styles	0.00%	0.00%	57,708	57,708	16,237	16,237
@material-ui/system	0.00%	0.00%	17,062	17,062	4,486	4,486
Button	0.00%	0.00%	99,068	99,068	26,483	26,483
Modal	+0.22% 🔺	+0.25% 🔺	98,649	98,863	26,151	26,217
colorManipulator	0.00%	0.00%	3,232	3,232	1,297	1,297
docs.landing	0.00%	0.00%	49,899	49,899	10,728	10,728
docs.main	+0.03% 🔺	+0.03% 🔺	677,788	678,005	205,910	205,975
packages/material-ui/build/umd/material-ui.production.min.js	+0.07% 🔺	+0.08% 🔺	319,832	320,053	84,603	84,670

Generated by 🚫 dangerJS against e9c1172