Prevent FormOptions from throwing an exception with the enhanced changes list

This threw an excpetion as $opts only knew about the hideWikibase option
if the enhanced changes list is disabled, but in SpecialWatchlistQueryHandler
we were querying for that option unconditionally.

Bug: 63087
Change-Id: I7e66550787d359e294b288e4c966e11fec1bcef1
(cherry picked from commit b139241fefb88ba80e9ec820eda839d1829e95b2)