Allow null flutter schema in pubspec (#15690)

diff --git a/packages/flutter_tools/test/flutter_manifest_test.dart b/packages/flutter_tools/test/flutter_manifest_test.dart
index ed7feb9..dca198a 100644
--- a/packages/flutter_tools/test/flutter_manifest_test.dart
+++ b/packages/flutter_tools/test/flutter_manifest_test.dart
@@ -345,5 +345,17 @@
       final List<Font> fonts = flutterManifest.fonts;
       expect(fonts.length, 0);
     });
+
+    test('allows a blank flutter section', () async {
+      const String manifest = '''
+name: test
+dependencies:
+  flutter:
+    sdk: flutter
+flutter:
+''';
+      final FlutterManifest flutterManifest = await FlutterManifest.createFromString(manifest);
+      expect(flutterManifest.isEmpty, false);
+    });
   });
 }