@Nullable
  private static List<String> addError(@Nullable List<String> errors, String newError) {
    if (newError == null) {
      return errors;
    }
    if (errors == null) {
      return getMutableSingletonList(newError);
    }

    Collections.addAll(errors, newError);

    return errors;
  }
Combines two {@literal List<String>s} together.
jobdispatcher/src/main/java/com/firebase/jobdispatcher/DefaultJobValidator.java