code: public StrBuilder setNullText(String nullText) {
          if (nullText != null && nullText.isEmpty()) {
              nullText = null;
          }
          this.nullText = nullText;
          return this;
      }
docs: Sets the text to be appended when null is added.
  
  @param nullText  the null text, null means no append
  @return this, to enable chaining