Make LinkCache fail more gracefully when given undefined

If you tried to do styleElement( undefined, $element ), you'd
get an exception in the mw.Title constructor from a setTimeout.
That's not very nice, and there's no point sending non-strings
through the queue and other data structures in LinkCache, so
instead just make get() return a rejected promise when given
non-string values.

Change-Id: Iab06929d7e232a8ecd8eb1eff9d1190c303421c1
Merge "Make LinkCache fail more gracefully when given undefined"