WatchedItemStore::countVisitingWatchersMultiple() shouldn't query all titles when asked for none
If a caller gives an empty array for $targetsWithVisitThresholds, per
the documentation it should be expecting an empty array in return, not a
count of watchers for every title in the database.
Bug: T204729
Change-Id: I0f25fae301450d077bb30597281aaef0fba209d4
(cherry picked from commit f5469d36602cb2a95396830b14e9a631d698f3a6)