fix(issue:3737) allow blank variable declarations

[puckowski]

What:

Fixes issue #3737 where blank CSS variable declarations resulted in parsing errors.

Why:

Blank CSS variables are valid.

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Existing tests are passing locally.

The following Less.js:

*, ::before, ::after {
  --tw-pan-x:  ;
  --tw-pan-y:  ;
  --tw-pinch-zoom:  ;
  --tw-scroll-snap-strictness: proximity;
}

becomes:

*,
::before,
::after {
  --tw-pan-x: ;
  --tw-pan-y: ;
  --tw-pinch-zoom: ;
  --tw-scroll-snap-strictness: proximity;
}

[puckowski]

I submitted a few PRs. I expect rebase(s) will be needed in order to merge all of them, so let me know when a rebase is needed.

[matthew-dean]

@puckowski Looks like we have a merge conflict now

[puckowski]

Tests passing locally. CI looks flaky.

[puckowski]

Rebased after merge conflict. Changes approved by one reviewer. Squashing and merging to avoid future merge conflicts.