Sorting "diff" column requires 2 clicks

On page like http://deadlock.netbeans.org/hudson/job/nbms-and-javadoc/4773/testReport/ , soring on the "diff" column first shows all blank values at the top, which is useless.

Maybe in this context, ignoring 0 for the sake of sorting (and always show them at the end) might be appropriate, since we don't show value 0.