Allow $context->setTitle( null )

If something is messing around with $wgTitle before MediaWiki::main()
sets $wgTitle in the first place, it needs to be able to reset it back
to null to avoid a problem like bug 53498. And to do that properly, it
also needs to be able to clear the title in the RequestContext.

Bug: 55435
Change-Id: I6c763b70bbffa0762ca52e61acf7dc200da4075f
Merge "Allow $context->setTitle( null )"