Fix a crash when UILaunchStoryboardName is not specified in Info.plist (#6234)

Some developers might prefer launch images.

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -392,6 +392,9 @@
   if (_splashScreenView == nullptr) {
     NSString* launchStoryboardName =
         [[[NSBundle mainBundle] infoDictionary] objectForKey:@"UILaunchStoryboardName"];
+    if (launchStoryboardName == nil) {
+      return nil;
+    }
     UIStoryboard* storyboard = [UIStoryboard storyboardWithName:launchStoryboardName bundle:nil];
     if (storyboard == nil) {
       return nil;