Fix empty adaptive text selection toolbars building. (flutter#174656)

Fixes flutter#174649

This PR fixes `AdaptiveTextSelectionToolbar` and
`CupertinoAdaptiveTextSelectionToolbar` building when `children` and
`buttonItems` are both `null`.

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Author: ksokolovskyi

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

@@ -209,7 +209,7 @@ class CupertinoAdaptiveTextSelectionToolbar extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     // If there aren't any buttons to build, build an empty toolbar.
-    if ((children?.isEmpty ?? false) || (buttonItems?.isEmpty ?? false)) {
+    if ((children ?? buttonItems)?.isEmpty ?? true) {
       return const SizedBox.shrink();
     }
 

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

@@ -297,7 +297,7 @@ class AdaptiveTextSelectionToolbar extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     // If there aren't any buttons to build, build an empty toolbar.
-    if ((children != null && children!.isEmpty) || (buttonItems != null && buttonItems!.isEmpty)) {
+    if ((children ?? buttonItems)?.isEmpty ?? true) {
       return const SizedBox.shrink();
     }
 

packages/flutter/test/cupertino/adaptive_text_selection_toolbar_test.dart

@@ -270,4 +270,25 @@ void main() {
         expect(find.byType(CupertinoDesktopTextSelectionToolbarButton), findsOneWidget);
     }
   }, variant: TargetPlatformVariant.all());
+
+  testWidgets(
+    'Builds empty toolbar when children and buttonItems are null',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const CupertinoApp(
+          home: Center(
+            child: CupertinoAdaptiveTextSelectionToolbar(
+              anchors: TextSelectionToolbarAnchors(primaryAnchor: Offset.zero),
+              children: null,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byType(CupertinoAdaptiveTextSelectionToolbar)), Size.zero);
+      expect(tester.takeException(), isNull);
+    },
+    skip: isBrowser, // [intended] see https://github.com/flutter/flutter/issues/108382
+    variant: TargetPlatformVariant.all(),
+  );
 }

packages/flutter/test/material/adaptive_text_selection_toolbar_test.dart

@@ -270,6 +270,27 @@ void main() {
     variant: TargetPlatformVariant.all(),
   );
 
+  testWidgets(
+    'Builds empty toolbar when children and buttonItems are null',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Center(
+            child: AdaptiveTextSelectionToolbar(
+              anchors: TextSelectionToolbarAnchors(primaryAnchor: Offset.zero),
+              children: null,
+            ),
+          ),
+        ),
+      );
+
+      expect(tester.getSize(find.byType(AdaptiveTextSelectionToolbar)), Size.zero);
+      expect(tester.takeException(), isNull);
+    },
+    skip: isBrowser, // [intended] on web the browser handles the context menu.
+    variant: TargetPlatformVariant.all(),
+  );
+
   group('buttonItems', () {
     testWidgets(
       'getEditableTextButtonItems builds the correct button items per-platform',