include nulls_first, nulls_last in order_by_label_element

include nulls_first, nulls_last in order_by_label_element

Fixed bug where the :meth:`.Operators.nulls_first()` and
:meth:`.Operators.nulls_last()` modifiers would not be treated the same way
as :meth:`.Operators.desc()` and :meth:`.Operators.asc()` when determining
if an ORDER BY should be against a label name already in the statement. All
four modifiers are now treated the same within ORDER BY.

Fixes: #11592
Change-Id: I1de1aff679c56af1abfdfd07f9bcbc45ecc5a8cc