code: public static TreeString of(final String s) {
          if (s == null) {
              return null;
          }
          return new TreeString(null, s);
      }
docs: Creates a {@link TreeString}. Useful if you need to create one-off
  {@link TreeString} without {@link TreeStringBuilder}. Memory consumption
  is still about the same to {@code new String(s)}.
  
  @return null if the parameter is null