code: public void addToStringCondition(SourceBuilder code) {
      checkState(initialState() == Initially.OPTIONAL);
      code.add("%s != null", property.getField());
    }
docs: Adds condition statement for an initially optional property to be included in the toString
  output for the Value/Partial types.
  
  @throws IllegalStateException if {@link #initialState()} is not {@link Initially#OPTIONAL}