CSS: Return `undefined` for whitespace-only CSS variable values (#5120)

CSS: Return `undefined` for whitespace-only CSS variable values (#5120)

The spec requires that CSS variable values are trimmed. In browsers that do
this - mainly, Safari, but also Firefox if the value only has leading
whitespace - we currently return undefined; in other browsers, we return
an empty string as the logic to fall back to undefined happens before
trimming.

This commit adds another explicit callback to `undefined` to have it consistent
across browsers.

Also, more explicit comments about behaviors we need to work around in various
browsers have been added.

Closes gh-5120
Ref gh-5106