Address review feedback for UseUserAccessGroup

This commit addresses feedback from the pull request review for the
UseUserAccessGroup feature:

- Added a stub implementation for Android in `auth_android.cc`.
- Updated iOS implementation in `auth_ios.mm` to pass an empty NSString
  (`@""`) to the native SDK if an empty C++ string is provided.
  `nullptr` still results in `nil` being passed.
- Updated Doxygen comments in `auth.h` to reflect the iOS behavior for
  `nullptr` and empty string inputs more accurately.
- Investigated potential NSError codes for the underlying iOS SDK method.
  Confirmed that `FIRAuthErrorCodeKeychainError` is the most relevant documented
  error, and it is already correctly handled by the existing C++ error
  mapping.

Author: google-labs-jules[bot]

auth/src/android/auth_android.cc

@@ -676,5 +676,10 @@ void DisableTokenAutoRefresh(AuthData* auth_data) {}
 void InitializeTokenRefresher(AuthData* auth_data) {}
 void DestroyTokenRefresher(AuthData* auth_data) {}
 
+AuthError Auth::UseUserAccessGroup(const char* access_group) {
+  // This is an iOS-only feature. No-op on other platforms.
+  return kAuthErrorNone;
+}
+
 }  // namespace auth
 }  // namespace firebase

auth/src/include/firebase/auth.h

@@ -525,11 +525,16 @@ class Auth {
   /// If you are using iCloud keychain synchronization, you will need to call
   /// this method to set the user access group.
   ///
-  /// @param[in] access_group The user access group to use. Set to `nullptr` or
-  /// an empty string to use the default access group.
+  /// @param[in] access_group The user access group to use.
+  ///   - On iOS, if `nullptr` is provided, `nil` will be passed to the
+  ///     underlying SDK, typically resetting to the default or app's main
+  ///     access group.
+  ///   - On iOS, if an empty string (`""`) is provided, an empty `NSString`
+  ///     will be passed to the underlying SDK.
+  ///   - On other platforms, this parameter is ignored.
   ///
   /// @return `kAuthErrorNone` on success, or an AuthError code if an error
-  /// occurred.
+  /// occurred (iOS only).
   AuthError UseUserAccessGroup(const char* access_group);
 
  private:

auth/src/ios/auth_ios.mm

@@ -613,7 +613,7 @@ void DestroyTokenRefresher(AuthData *auth_data) {}
     return kAuthErrorUninitialized;
   }
   NSString* access_group_ns_str = nil;
-  if (access_group_str != nullptr && strlen(access_group_str) > 0) {
+  if (access_group_str != nullptr) {
     access_group_ns_str = [NSString stringWithUTF8String:access_group_str];
   }