Prevent Special:Diff from processing empty string

Umherirrender found that I77fdaf8e has a flaw. Currently it is possible
to go to index.php?title=Special:Diff (without any trailing string),
and the page will be redirected to index.php?diff= which is the last diff
of the main page. However, the intended way to deal with this case is to
raise ErrorPageError. This patch fixes the problem.

Change-Id: I3ff0b11671f27e77d9068930338ec8bbb674a89f