Disallow empty arbitrary values

[RobinMalfait]

This PR makes the candidate parser more strict by not allowing empty arbitrary values.

Examples that are not allowed anymore:

• bg-[] — arbitrary value
• bg-() — arbitrary value, var shorthand
• bg-[length:] — arbitrary value, with typehint
• bg-(length:) — arbitrary value, with typehint, var shorthand
• bg-red-500/[] — arbitrary modifier
• bg-red-500/() — arbitrary modifier, var shorthand
• data-[]:flex — arbitrary value for variant
• data-():flex — arbitrary value for variant, var shorthand
• group-visible/[]:flex — arbitrary modifier for variant
• group-visible/():flex — arbitrary modifier for variant, var shorthand

If you are trying to trick the parser by injecting some spaces like this:

• bg-[_]

Then that is also not allowed.