JsonNull return false when comparing against null (#842)

JsonNull return false when comparing against null (#842)

* JsonNull return false when comparing against null

I'ved modified JsonNulls comparison methods to return false when
compared against null, the matching test have also been updated
in change

Fix #820

* Updated xml comments to reflect my changes

I have updated the xml comments to reflect the changes
that was made in JsonNull and removed left over comment
in JsonNullTests

* Applied suggested change

Applied the suggested change to correct xml documentation

Co-Authored-By: Ahson Khan <ahkha@microsoft.com>
* Applied Suggested change

Applied the suggested change to correct xml documentation

Co-Authored-By: Ahson Khan <ahkha@microsoft.com>