Pretty obvious in retrospect. If there's no results, then we need to
preview
_nothing_ to make sure that we clear out any old previews.
Closes#17773
Additionally, while I was here:
I realized why it seems like the selected item is so wacky when you
first open the sxnui:
* on launch we're not scrolling to the bottom item (which makes it
awkward in bottom up mode)
* when we filter the list, we're maintaining the selection _index_, not
the selection _item_.
Alas, that second part is... shockingly bodgy.
1de142b4b1