Handle Edge Cases on NullValue.java - todo: "Sort empty Types" - Contributes to Issue #141

[ronaldngounou]

Hello,

As described in the issue related to handle edge cases on the issue #141 "sort empty types" , I would like to suggest an approach to solve this issue.

This PR updates the compareTo method in the NullValue class to clarify how "empty types" should be handled in comparisons. The TODO comment has been ameliorated by listing relevant class types (EmptyValue, ArrayValue, BinaryValue, BooleanValue, ... ) that could be null.

This attemps to clarify edge cases scenarios.