code: @Nullable
      public static String emptyToNull(@Nullable String string) {
          return isNullOrEmpty(string) ? null : string;
      }
docs: Returns the given string if it is nonempty; {@code null} otherwise.
  
  @param string the string to test and possibly return
  @return {@code string} itself if it is nonempty; {@code null} if it is empty or null