Handle null Darwin getTitleAsync results without crashing - #1391

PhotoManagerPlugin.getTitleAsync could receive a null result from the Darwin platform channel even though the Dart API returns Future<String>. This change removes the non-null assumption and falls back to an empty string instead.

• Runtime fix

  • Replace the implicit non-null expectation in getTitleAsync with an inlined null-safe fallback (?? '') for iOS/macOS results.
  • Keep non-Darwin behavior aligned by using the same fallback path for entity.title.

• Changelog

  • Document the fix under Unreleased.

if (Platform.isIOS || Platform.isMacOS) {
  return await _channel.invokeMethod<String>(
        PMConstants.mGetTitleAsync,
        <String, dynamic>{...},
      ) ??
      '';
}
return entity.title ?? '';