Fixing focus traversal when the node options are empty (#43238)

Fixes directional focus traversal when there are no available nodes to traverse to.

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

@@ -249,7 +249,11 @@ mixin DirectionalFocusTraversalPolicyMixin on FocusTraversalPolicy {
         }
       }
     });
-    return sorted.first;
+
+    if (sorted.isNotEmpty)
+      return sorted.first;
+
+    return null;
   }
 
   // Sorts nodes from left to right horizontally, and removes nodes that are

packages/flutter/test/widgets/focus_traversal_test.dart

@@ -1127,6 +1127,25 @@ void main() {
       await tester.sendKeyEvent(LogicalKeyboardKey.arrowUp);
       expect(focusNodeUpperLeft.hasPrimaryFocus, isTrue);
     });
+    testWidgets('Focus traversal does not break when no focusable is available on a MaterialApp',   (WidgetTester tester) async {
+      final List<RawKeyEvent> events = <RawKeyEvent>[];
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Container()
+        )
+      );
+
+      RawKeyboard.instance.addListener((RawKeyEvent event) {
+        events.add(event);
+      });
+
+      await tester.idle();
+      await tester.sendKeyEvent(LogicalKeyboardKey.arrowRight);
+      await tester.idle();
+
+      expect(events.length, 2);
+    });
   });
 }