Eliminate unnecessary null check in fixture

shell/platform/embedder/fixtures/main.dart

@@ -677,7 +677,7 @@ void can_display_platform_view_with_pixel_ratio() {
 @pragma('vm:entry-point')
 void can_receive_locale_updates() {
   PlatformDispatcher.instance.onLocaleChanged = (){
-    signalNativeCount(PlatformDispatcher.instance.locales!.length);
+    signalNativeCount(PlatformDispatcher.instance.locales.length);
   };
   signalNativeTest();
 }