code: public boolean isNullString(String input) {
          if (input == null) {
              throw new NullPointerException(makeNullArgumentErrorMsg("input"));
          }
          return nullStringStrategy.isNullString(input, new NullStringStrategyHelper(targetType));
      }
docs: Checks if the given <code>input</code> is considered a <code>NullString</code> or not.
  Returns true if it is, otherwise returns false.
  
  @param input string to parse.
  @return true if <code>input</code> is a <code>NullString</code>, otherwise false.
  @throws NullPointerException if given argument is {@code null}.
  @see NullStringStrategy