update CupertinoPicker.selectionOverlay to nullable (#79620)

packages/flutter/lib/src/cupertino/picker.dart

@@ -198,7 +198,7 @@ class CupertinoPicker extends StatefulWidget {
   /// If unspecified, it defaults to a [CupertinoPickerDefaultSelectionOverlay]
   /// which is a gray rounded rectangle overlay in iOS 14 style.
   /// This property can be set to null to remove the overlay.
-  final Widget selectionOverlay;
+  final Widget? selectionOverlay;
 
   @override
   State<StatefulWidget> createState() => _CupertinoPickerState();
@@ -301,7 +301,8 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
               ),
             ),
           ),
-          _buildSelectionOverlay(widget.selectionOverlay),
+          if (widget.selectionOverlay != null)
+            _buildSelectionOverlay(widget.selectionOverlay!),
         ],
       ),
     );

packages/flutter/test/cupertino/picker_test.dart

@@ -172,6 +172,29 @@ void main() {
     expect(find.byType(CupertinoPicker), paints..rrect(color: const Color(0x12345678)));
   });
 
+  testWidgets('CupertinoPicker.selectionOverlay is nullable', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        theme: const CupertinoThemeData(brightness: Brightness.light),
+        home: Align(
+          alignment: Alignment.topLeft,
+          child: SizedBox(
+            height: 300.0,
+            width: 300.0,
+            child: CupertinoPicker(
+              itemExtent: 15.0,
+              children: const <Widget>[Text('1'), Text('1')],
+              onSelectedItemChanged: (int i) {},
+              selectionOverlay: null,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byType(CupertinoPicker), isNot(paints..rrect()));
+  });
+
   group('scroll', () {
     testWidgets(
       'scrolling calls onSelectedItemChanged and triggers haptic feedback',