Discard candidates with an empty data type

[thecrypticace]

Fixes tailwindlabs/tailwindcss-intellisense#1479

Maybe should close tailwindlabs/tailwindcss-intellisense#1480 — perhaps we can find a workaround there for older versions?

We're building up a class name in code to validate if something is a valid variant: {variant}:[color:red]

if {variant} got replaced with bg-[ then we'd produce bg-[:[color:red] and this parsed as a valid candidate:

bg-[:[color:red]
^^                root: `bg`
   ^              data type: `` (empty string) — this should be invalid
     ^^^^^^^^^^   value: `[color:red`

The value isn't valid but the syntax for arbitrary values is pretty lax in core. Oxide already won't pick something like this up though so no problem there. Only a problem for something like IntelliSense or clients using the compile() API directly.

[RobinMalfait]

I think we can add a changelog entry for this one.