retrieveIfExists method
Return null only for a canonical clean miss; propagate every real error.
Implementation
String? retrieveIfExists(String key) {
final result = retrieve(key);
if (result.status > 0) return result.value;
if (result.status == RacResultCode.errorFileNotFound) return null;
SDKException.throwIfError(result.status);
throw StateError('Secure-storage read returned an invalid status');
}