Fixed 0003853: Filters sometime generate an invalid query (partial fix).

M core/filter_api.php

• If sort field is empty default it to last_updated and direction to DESC. The root problem still has to be fixed, but this fixes the symptoms. This is also needed to avoid SQL errors for filters saved in user dbs.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2551 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9