SpecialEditWatchlist: Avoid empty namespace sections

SpecialEditWatchlist: Avoid empty namespace sections

When the database contains a title for a positive namespace which is no
longer active, a table of contents item and a empty section is shown on
Special:EditWatchlist. After reload the section is gone, because the
database was cleaned up and the invalid namespace can no longer produce
the section. Refactor a bit to avoid the empty section on the first
view as well to get a stable output of that special page.

Change-Id: I41426bde71b21a4abddb12af0b3a84931f51ac97