fix: Address review comments for UseUserAccessGroup

This commit incorporates feedback for the `UseUserAccessGroup` method:

- Stub implementations for Android and desktop now return
  `kAuthErrorUnimplemented` instead of `kAuthErrorNone`.
- The iOS implementation now returns `kAuthErrorUninitialized`
  if `auth_data_` is null.
- The iOS implementation now treats an empty string for `access_group`
  the same as `nullptr` (passing `nil` to the Objective-C method).

Author: google-labs-jules[bot]

auth/src/android/auth_android.cc

@@ -671,8 +671,8 @@ void Auth::UseEmulator(std::string host, uint32_t port) {
 }
 
 AuthError Auth::UseUserAccessGroup(const char* access_group) {
-  // No-op on Android.
-  return kAuthErrorNone;
+  // Not implemented on Android.
+  return kAuthErrorUnimplemented;
 }
 
 // Not implemented for Android.

auth/src/desktop/auth_desktop.cc

@@ -576,8 +576,8 @@ void Auth::UseEmulator(std::string host, uint32_t port) {
 }
 
 AuthError Auth::UseUserAccessGroup(const char* access_group) {
-  // No-op on desktop.
-  return kAuthErrorNone;
+  // Not implemented on desktop.
+  return kAuthErrorUnimplemented;
 }
 
 void InitializeTokenRefresher(AuthData* auth_data) {

auth/src/ios/auth_ios.mm

@@ -592,11 +592,11 @@ void SignInCallback(FIRUser *_Nullable user, NSError *_Nullable error,
 
 AuthError Auth::UseUserAccessGroup(const char* access_group) {
   if (!auth_data_) {
-    return kAuthErrorNone; // Or appropriate error if auth_data_ is unexpectedly null
+    return kAuthErrorUninitialized;
   }
   FIRAuth* fir_auth = AuthImpl(auth_data_);
   NSString* ns_access_group = nil;
-  if (access_group) {
+  if (access_group && strlen(access_group) > 0) {
     ns_access_group = [NSString stringWithUTF8String:access_group];
   }