Fix: checking for filter_string!='' breaks mssql as filter_string is a 'text' field. We shouldn't be storing empty filter's into db anyway...

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@3273 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9