Add checks on auth_data, because of DeleteInternal

The new logic of DeleteInternal means that all the auth functions need to handle that it has been deleted.
Also update the Unity automated test, so that it works.

Tested by:
Ran the Unity automated test on Mac editor

PiperOrigin-RevId: 238766343

Author: a-maurice

auth/src/auth.cc

@@ -167,14 +167,18 @@ Auth::~Auth() { DeleteInternal(); }
 // holds nothing but a pointer to AuthData, which never changes.
 // All User functions that require synchronization go through AuthData's mutex.
 User* Auth::current_user() {
+  if (!auth_data_) return nullptr;
   MutexLock lock(auth_data_->future_impl.mutex());
   User* user =
       auth_data_->user_impl == nullptr ? nullptr : &auth_data_->current_user;
   return user;
 }
 
 // Always non-nullptr since set in constructor.
-App& Auth::app() { return *auth_data_->app; }
+App& Auth::app() {
+  FIREBASE_ASSERT(auth_data_ != nullptr);
+  return *auth_data_->app;
+}
 
 template <typename T>
 static bool PushBackIfMissing(const T& entry, std::vector<T>* v) {
@@ -205,11 +209,13 @@ static void AddListener(T listener, std::vector<T>* listener_vector, Auth* auth,
 }
 
 void Auth::AddAuthStateListener(AuthStateListener* listener) {
+  if (!auth_data_) return;
   AddListener(listener, &auth_data_->listeners, this, &listener->auths_,
               &auth_data_->listeners_mutex);
 }
 
 void Auth::AddIdTokenListener(IdTokenListener* listener) {
+  if (!auth_data_) return;
   int listener_count = auth_data_->id_token_listeners.size();
   AddListener(listener, &auth_data_->id_token_listeners, this,
               &listener->auths_, &auth_data_->listeners_mutex);
@@ -253,11 +259,13 @@ static void RemoveListener(T listener, std::vector<T>* listener_vector,
 }
 
 void Auth::RemoveAuthStateListener(AuthStateListener* listener) {
+  if (!auth_data_) return;
   RemoveListener(listener, &auth_data_->listeners, this, &listener->auths_,
                  &auth_data_->listeners_mutex);
 }
 
 void Auth::RemoveIdTokenListener(IdTokenListener* listener) {
+  if (!auth_data_) return;
   int listener_count = auth_data_->id_token_listeners.size();
   RemoveListener(listener, &auth_data_->id_token_listeners, this,
                  &listener->auths_, &auth_data_->listeners_mutex);