Fix AlertDialog.adaptive clipBehavior is non-nullable (#154173)

Fixes flutter/flutter#153983

Author: TahaTesser

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

@@ -464,7 +464,7 @@ class AlertDialog extends StatelessWidget {
     Color? surfaceTintColor,
     String? semanticLabel,
     EdgeInsets insetPadding,
-    Clip clipBehavior,
+    Clip? clipBehavior,
     ShapeBorder? shape,
     AlignmentGeometry? alignment,
     bool scrollable,

packages/flutter/test/material/dialog_test.dart

@@ -2894,6 +2894,12 @@ void main() {
     const SimpleDialog simpleDialog = SimpleDialog();
     expect(simpleDialog.insetPadding, isNull);
   });
+
+  testWidgets('Can pass a null value to AlertDialog.adaptive clip behavior', (WidgetTester tester) async {
+    for (final Clip? clipBehavior in <Clip?>[null, ...Clip.values]) {
+      AlertDialog.adaptive(clipBehavior: clipBehavior);
+    }
+  });
 }
 
 class _RestorableDialogTestWidget extends StatelessWidget {