core: Search query sortby return some wrongly...

Search query sortby return some wrongly sorted results

Adding explicitly "NULLS LAST" when DESC ORDER BY is used

This is the ORDER BY Syntax :

    ORDER BY sort_expression1 [ASC | DESC] [NULLS { FIRST | LAST }]
     [, sort_expression2 [ASC | DESC] [NULLS { FIRST | LAST }] ...]

The NULLS FIRST and NULLS LAST options can be used to determine whether
nulls appear before or after non-null values in the sort ordering. By
default, null values sort as if larger than any non-null value; that is,
NULLS FIRST is the default for DESC order, and NULLS LAST otherwise.

Change-Id: Ib1f25076ffe7c0c6e1dbd609f7b175c8c9e34ac5
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=962560
Signed-off-by: Eli Mesika <emesika@redhat.com>