don't bother searching users at all without search term

without this, the page could hang(query could take a lot of time) if users
number is huge.  (I got bitten with 700,000 users before)

technically, when there is no $UserString, though RT::Users::SimpleSearch
directly returns, the following $users calls like "$users->First", etc will
still query the db, which is both time-consuming and unnecessary in this case.
Merge branch '4.2/do-not-search-without-term-in-user-search' into 4.2-trunk