Fix the behavior of multi-sort. All the nil values are appended at the end of
the result irrespective of ascending or descending sort. This change also
fixes a bug due to not appending the nil values in the values list,
corresponding to the UIDs appended in the UID list with nil predicates.

(cherry picked from commit 2c7de573090f700841b2d9bda2779c48349fedc5)