Fix MaterialApp's _navigatorObserver when only builder used (#29413)

Fix MaterialApp's _navigatorObserver so it doesn't cause WidgetApp's assertion to fail when using custom navigator, fixes #18904

packages/flutter/lib/src/material/app.dart

@@ -419,7 +419,7 @@ class _MaterialAppState extends State<MaterialApp> {
       _navigatorObservers = List<NavigatorObserver>.from(widget.navigatorObservers)
         ..add(_heroController);
     } else {
-      _navigatorObservers = null;
+      _navigatorObservers = const <NavigatorObserver>[];
     }
   }
 

packages/flutter/test/material/app_test.dart

@@ -381,6 +381,17 @@ void main() {
     expect(log, <String>['onGenerateRoute /', 'onUnknownRoute /']);
   });
 
+  testWidgets('MaterialApp with builder and no route information works.', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/18904
+    await tester.pumpWidget(
+      MaterialApp(
+        builder: (BuildContext context, Widget child) {
+          return const SizedBox();
+        },
+      ),
+    );
+  });
+
   testWidgets('Can get text scale from media query', (WidgetTester tester) async {
     double textScaleFactor;
     await tester.pumpWidget(MaterialApp(