TextStyle: In copyWith, stop ignoring debugLabel when receiver has none (#141141)

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.

packages/flutter/lib/src/painting/text_style.dart

@@ -893,8 +893,10 @@ class TextStyle with Diagnosticable {
     assert(backgroundColor == null || background == null, _kColorBackgroundWarning);
     String? newDebugLabel;
     assert(() {
-      if (this.debugLabel != null) {
-        newDebugLabel = debugLabel ?? '(${this.debugLabel}).copyWith';
+      if (debugLabel != null) {
+        newDebugLabel = debugLabel;
+      } else if (this.debugLabel != null) {
+        newDebugLabel = '(${this.debugLabel}).copyWith';
       }
       return true;
     }());

packages/flutter/test/painting/text_style_test.dart

@@ -374,6 +374,7 @@ void main() {
     expect(unknown.debugLabel, null);
     expect(unknown.toString(), 'TextStyle(<all styles inherited>)');
     expect(unknown.copyWith().debugLabel, null);
+    expect(unknown.copyWith(debugLabel: '123').debugLabel, '123');
     expect(unknown.apply().debugLabel, null);
 
     expect(foo.debugLabel, 'foo');