Fix: Remove usage of ?? operator

[nmn]

?? isn't supported in old browsers, and its usage is breaking in old versions of Chrome.

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 2.36 kB

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Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/stylex.js	1.02 (3.53)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.34 (3.41)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1119.39 kB

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Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	1005.22 (9872.52)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	114.18 (455.95)	0.00 (0.00)	0.0% (0.0%)