Correctly handle incorrect namespace in cleanupTitles.php

If the namespace is not valid; Title::makeTitleSafe() will
return null, thus producing a fatal error. Work arround this
by setting the namespace to 0 in that case.

Bug: 68501
Change-Id: I0c22f9468ff2bf11d2bf4a9265fa454ece2c0fa3
Merge "Correctly handle incorrect namespace in cleanupTitles.php"