[ButtonGroup] Fix missing divider if background color is set

[neon98]

• I have followed (at least) the PR section of the contributing guide.

fixes #17495

[mui-pr-bot]

Details of bundle changes.

Comparing: 05c533b...7919532

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.01% 🔺	0.00%	332,538	332,562	90,732	90,732
@material-ui/core/Paper	0.00%	0.00%	68,935	68,935	20,541	20,541
@material-ui/core/Paper.esm	0.00%	0.00%	62,372	62,372	19,275	19,275
@material-ui/core/Popper	0.00%	0.00%	28,405	28,405	10,172	10,172
@material-ui/core/Textarea	0.00%	0.00%	5,082	5,082	2,136	2,136
@material-ui/core/TrapFocus	0.00%	0.00%	3,766	3,766	1,596	1,596
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,348	16,348	5,818	5,818
@material-ui/core/useMediaQuery	0.00%	0.00%	2,488	2,488	1,050	1,050
@material-ui/lab	0.00%	0.00%	143,405	143,405	43,808	43,808
@material-ui/styles	0.00%	0.00%	51,694	51,694	15,364	15,364
@material-ui/system	0.00%	0.00%	15,581	15,581	4,341	4,341
Button	0.00%	0.00%	78,772	78,772	24,043	24,043
Modal	0.00%	0.00%	14,542	14,542	5,113	5,113
Portal	0.00%	0.00%	2,907	2,907	1,318	1,318
Rating	0.00%	0.00%	70,238	70,238	21,929	21,929
Slider	0.00%	0.00%	75,379	75,379	23,260	23,260
colorManipulator	0.00%	0.00%	3,835	3,835	1,519	1,519
docs.landing	0.00%	0.00%	54,267	54,267	14,344	14,344
docs.main	0.00%	0.00%	588,062	588,062	188,043	188,043
packages/material-ui/build/umd/material-ui.production.min.js	+0.01% 🔺	-0.00%	303,356	303,380	87,276	87,275

Generated by 🚫 dangerJS against 7919532

[oliviertassinari]

Thanks! We leave 1 week for people that claim an issue, it seems that the delay has just expired in your favor for this one :).

[neon98]

haha! yes 😄