Make sure that a CupertinoDesktopTextSelectionToolbar doesn't crash i… (flutter#173964)

This is my attempt to handle
flutter#6537 for the
CupertinoDesktopTextSelectionToolbar UI control.

---------

Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>

Author: ahmedsameha1

packages/flutter/test/cupertino/desktop_text_selection_toolbar_test.dart

@@ -127,4 +127,22 @@ void main() {
       greaterThan(anchor),
     );
   });
+
+  testWidgets('CupertinoDesktopTextSelectionToolbar does not crash at zero area', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: SizedBox.shrink(
+            child: CupertinoDesktopTextSelectionToolbar(
+              anchor: const Offset(10, 10),
+              children: const <Widget>[Text('X')],
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(tester.getSize(find.byType(CupertinoDesktopTextSelectionToolbar)), Size.zero);
+  });
 }