[flutter] Allow ViewCollection to start empty. (#140532)

This enables multi-view apps to start with zero views.

packages/flutter/lib/src/widgets/view.dart

@@ -475,9 +475,7 @@ class _MultiChildComponentWidget extends Widget {
 ///  * [WidgetsBinding] for an explanation of rendering and non-rendering zones.
 class ViewCollection extends _MultiChildComponentWidget {
   /// Creates a [ViewCollection] widget.
-  ///
-  /// The provided list of [views] must contain at least one widget.
-  const ViewCollection({super.key, required super.views}) : assert(views.length > 0);
+  const ViewCollection({super.key, required super.views});
 
   /// The [View] descendants of this widget.
   ///

packages/flutter/test/widgets/tree_shape_test.dart

@@ -293,8 +293,8 @@ void main() {
   testWidgets('ViewCollection cannot have render object widgets as children', (WidgetTester tester) async {
     await pumpWidgetWithoutViewWrapper(
       tester: tester,
-      widget: ViewCollection(
-        views: const <Widget>[
+      widget: const ViewCollection(
+        views: <Widget>[
           ColoredBox(color: Colors.red),
         ],
       ),

packages/flutter/test/widgets/view_test.dart

@@ -140,8 +140,8 @@ void main() {
     );
   });
 
-  testWidgets('ViewCollection must have one view', (WidgetTester tester) async {
-    expect(() => ViewCollection(views: const <Widget>[]), throwsAssertionError);
+  testWidgets('ViewCollection may start with zero views', (WidgetTester tester) async {
+    expect(() => const ViewCollection(views: <Widget>[]), returnsNormally);
   });
 
   testWidgets('ViewAnchor.child does not see surrounding view', (WidgetTester tester) async {