code: public String getString(String propertyName) {
          Object obj = get(propertyName);
          return obj != null ? obj.toString() : null;
      }
docs: Gets the value of the property as a {@code String}.
  This will use {@link Object#toString()}.
  <p>
  This returns null if the property does not exist.
  
  @param propertyName  the property name, not empty
  @return the value of the property, may be null