Ensure `--value(…)` is required in functional `@utility` definitions (#20005)

While working on #19989, I noticed that `--value(…)` inside functional
`@utility` definitions is not required right now.

That means that the following CSS is valid:
```css
@utility foo-* {
  color: red;
}
```

But this doesn't really makes sense, because this now accepts a value
and `foo-a`, `foo-b` and `foo-c` would generate the following CSS:
```css
.foo-a {
  color: red;
}
.foo-b {
  color: red;
}
.foo-c {
  color: red;
}
```
The `a`, `b`, and `c` are not doing anything here apart from making your
CSS bigger. So this is very likely an actual bug that you forgot to use
`--value(…)`.

Additionally, if a `--value(…)` was used, but it didn't resolve
anything, then we already properly discared the candidate.

## Test plan

1. Add test to ensure `--value(…)` is required in functional `@utility`
definitions
2. Existing tests pass