context: Support setting title to null

context: Support setting title to null

We have various entry points without a title set:

* load.php: Setup.php defaults $wgTitle to null. When messages
  are parsed, MessageCache::parse() will emit a warning and uses
  $wgTitle which is then casted to Special:Badtitle.

* Parser tests: Work all with context now (or parser options)
  without ever setting $wgTitle. However, in order to allow SpecialPageFactory
  to getTitle() and then setTitle() the same value from context, context needs
  to support setting null as title, the same way it can get null as title.

Change-Id: I770f449fe67beedc3f7d4009ed2208e9dde4491c