Fix helper_mantis_url() to handle blank strings for use with manage menus and others.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4829 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9