fix(auth, apple): fetchSignInMethodsForEmail if value is nil, pass empty array. (#11596)

* fix(auth, apple): `fetchSignInMethodsForEmail` if value is `nil`, pass empty array

* chore(auth, apple): format code

Author: russellwheatley

packages/firebase_auth/firebase_auth/ios/Classes/FLTFirebaseAuthPlugin.m

@@ -1049,7 +1049,11 @@ - (void)fetchSignInMethodsForEmailApp:(nonnull PigeonFirebaseApp *)app
                           if (error != nil) {
                             completion(nil, [FLTFirebaseAuthPlugin convertToFlutterError:error]);
                           } else {
-                            completion(providers, nil);
+                            if (providers == nil) {
+                              completion(@[], nil);
+                            } else {
+                              completion(providers, nil);
+                            }
                           }
                         }];
 }