Create an InlineSpanSemanticsInformation only if the TextSpan has text (#75666)

Fixes https://github.com/flutter/flutter/issues/75622

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

@@ -71,7 +71,8 @@ class TextSpan extends InlineSpan {
     TextStyle? style,
     this.recognizer,
     this.semanticsLabel,
-  }) : super(style: style);
+  }) : assert(!(text == null && semanticsLabel != null)),
+       super(style: style);
 
   /// The text contained in this span.
   ///
@@ -279,7 +280,7 @@ class TextSpan extends InlineSpan {
   @override
   void computeSemanticsInformation(List<InlineSpanSemanticsInformation> collector) {
     assert(debugAssertIsValid());
-    if (text != null || semanticsLabel != null) {
+    if (text != null) {
       collector.add(InlineSpanSemanticsInformation(
         text!,
         semanticsLabel: semanticsLabel,

packages/flutter/test/painting/text_span_test.dart

@@ -231,4 +231,11 @@ void main() {
     expect(textSpan.getSpanForPosition(const TextPosition(offset: 2)).runtimeType, WidgetSpan);
     expect(textSpan.getSpanForPosition(const TextPosition(offset: 3)).runtimeType, TextSpan);
   });
+
+  test('TextSpan computeSemanticsInformation', () {
+    final List<InlineSpanSemanticsInformation> collector = <InlineSpanSemanticsInformation>[];
+    const TextSpan(text: 'aaa', semanticsLabel: 'bbb').computeSemanticsInformation(collector);
+    expect(collector[0].text, 'aaa');
+    expect(collector[0].semanticsLabel, 'bbb');
+  });
 }