logging: Remove 'null' return from ManualLogEntry::getTags()

logging: Remove 'null' return from ManualLogEntry::getTags()

The documentation already claimed it returned array,
this is now actually true. Follows-up 037c06e886c63.

There are two ways it could be null:

* The setTags() method was never called, and the class member
  was implicitly null by default.

* The setTags() method was called with $tags = null.
  This happens WikiPage::doUpdateRestrictions() among other places.
  That could probably be fixed, but is out of scope for this change.

The getTags() method of this clss appears to have no outside
callers in MediaWiki core, nor in any other project indexed
by Codesearch.

Change-Id: Ib89c79ef90870506187c3c3b61464c2aa788c594