fix: handle empty strings in mergeClasses()

[layershifter]

A fix for a problem that I discovered in microsoft/fluentui#25075.

After #207 we don't emit useless sequence hashes that should improve performance of classes merging in such cases. The accidental problem that came with it:

It's happening because we handle empty strings as valid classes and add spacing between them, i.e.:

mergeClasses('', '', '') // "  " (a string from two spaces)

This PR fixes that problem.

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
core makeStyles + mergeClasses (build time)	1.834 kB 872 B	1.842 kB 877 B	8 B 5 B
core makeStyles + mergeClasses (runtime)	20.781 kB 7.72 kB	20.789 kB 7.724 kB	8 B 4 B
react __css + mergeClasses (build time)	1.871 kB 869 B	1.879 kB 874 B	8 B 5 B
react __styles + mergeClasses (build time)	3.573 kB 1.609 kB	3.581 kB 1.614 kB	8 B 5 B
react makeStyles + mergeClasses (runtime)	22.546 kB 8.432 kB	22.554 kB 8.436 kB	8 B 4 B

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
core makeResetStyles (runtime)	16.192 kB 6.041 kB
react makeResetStyles (runtime)	17.957 kB 6.777 kB
react makeStaticStyles (runtime)	9.443 kB 4.068 kB

_{🤖 This report was generated against d0511fc7844a552954f59c2f55f94e1ca26b013b}