Properly resolve theme('someKey.DEFAULT') when only --some-key-* keys exist

[adamwathan]

This PR fixes an issue where theme function calls like theme('transitionTimingFunction.DEFAULT') would incorrectly resolve to an object when the set of defined CSS theme values looked like this:

@theme {
  --transition-timing-function-in: ease-in;
  --transition-timing-function-out: ease-out;
  --transition-timing-function-in-out: ease-out;
}

We were mistakenly retrieving the entire --transition-timing-function-* namespace in this case and returning an object, even though the user is explicitly asking for a single value by including .DEFAULT in their call.

This ensures it resolves to null instead. Fixes an issue I ran into on this live stream earlier today:

https://x.com/adamwathan/status/1831740214051799281