[Transition] Fix false-positive ref warning

[eps1lon]

Fix false-positive ref warning with suggestion from @oliviertassinari
Closes #15515

useForkRef always created a callback even if none of the branches required a ref leading to false positives when the component didn't need the ref but just forwarded the ref to a clones child.

Used this opportunity to improve tests for useForkRef. Especially ref cleanup was never actually tested.

[mui-pr-bot]

Details of bundle changes.

Comparing: 8c5a1ca...108456c

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.01% 🔺	+0.02% 🔺	311,394	311,431	84,601	84,619
@material-ui/core/Paper	0.00%	0.00%	67,623	67,623	20,124	20,124
@material-ui/core/Paper.esm	0.00%	0.00%	60,988	60,988	19,020	19,020
@material-ui/core/Popper	+0.12% 🔺	+0.10% 🔺	31,114	31,151	10,803	10,814
@material-ui/core/Textarea	+0.68% 🔺	+0.72% 🔺	5,468	5,505	2,365	2,382
@material-ui/core/TrapFocus	+0.99% 🔺	+0.58% 🔺	3,731	3,768	1,565	1,574
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	15,943	15,943	5,777	5,777
@material-ui/core/useMediaQuery	0.00%	0.00%	2,106	2,106	974	974
@material-ui/lab	+0.03% 🔺	+0.04% 🔺	140,997	141,034	42,651	42,666
@material-ui/styles	0.00%	0.00%	51,151	51,151	15,148	15,148
@material-ui/system	0.00%	0.00%	11,765	11,765	3,923	3,923
Button	+0.04% 🔺	+0.06% 🔺	85,901	85,938	25,732	25,747
Modal	+0.18% 🔺	+0.14% 🔺	20,575	20,612	6,603	6,612
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	51,486	51,486	11,342	11,342
docs.main	+0.01% 🔺	+0.01% 🔺	648,595	648,632	202,362	202,378
packages/material-ui/build/umd/material-ui.production.min.js	+0.01% 🔺	+0.01% 🔺	293,109	293,146	82,525	82,535

Generated by 🚫 dangerJS against 108456c

[oliviertassinari]

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