code: public static <T> PositionFactory<Object, T> optional(String propertyName) {
          final boolean optional = true;
          return new PositionFactory<>(propertyName, optional);
      }
docs: Create a {@link Property.PositionFactory} for an optional property. A position created from such a factory will
  silently return {@code null} as its value if its parent's value is {@code null}.
  
  @param propertyName
  @return {@link PositionFactory}