Fixes #141140.

This ensures that if you call `.copyWith` and pass a `debugLabel`, the `debugLabel` won't be ignored, even if the receiver (the TextStyle you're calling `.copyWith` on) doesn't have a `debugLabel`.

The debugLabel field was added in #12552. I skimmed the discussion there and didn't find anything indicating that the param was being ignored on purpose.

I added a test case that passes with the new code and fails with the old code.