Add support for `Future<List<int>?>` to `MatchesGoldenFile` (#132965)

Otherwise when tests use `expectLater(getBytesOrNull(), matchesGoldenFile(...))`, they may fail in sound null safety mode and pass in weak null safety mode.

Fixes https://github.com/flutter/flutter/issues/132964

packages/flutter_test/lib/src/_matchers_io.dart

@@ -60,8 +60,9 @@ class MatchesGoldenFile extends AsyncMatcher {
     final Uri testNameUri = goldenFileComparator.getTestUri(key, version);
 
     Uint8List? buffer;
-    if (item is Future<List<int>>) {
-      buffer = Uint8List.fromList(await item);
+    if (item is Future<List<int>?>) {
+      final List<int>? bytes = await item;
+      buffer = bytes == null ? null : Uint8List.fromList(bytes);
     } else if (item is List<int>) {
       buffer = Uint8List.fromList(item);
     }

packages/flutter_test/test/matchers_test.dart

@@ -470,6 +470,14 @@ void main() {
         expect(comparator.imageBytes, equals(<int>[1, 2]));
         expect(comparator.golden, Uri.parse('foo.png'));
       });
+
+      testWidgets('future nullable list of integers',
+          (WidgetTester tester) async {
+        await expectLater(Future<List<int>?>.value(<int>[1, 2]), matchesGoldenFile('foo.png'));
+        expect(comparator.invocation, _ComparatorInvocation.compare);
+        expect(comparator.imageBytes, equals(<int>[1, 2]));
+        expect(comparator.golden, Uri.parse('foo.png'));
+      });
     });
 
     group('does not match', () {