core: Protect against system server take down on empty tile order.

  If a user, for whatever reason, removes all the tiles available
  in the QSPanel, any query to #isQSTileEnabledForUser would result
  in an NPE. With the introduction of Dynamic Tiles, this issue
  is exacerbated causing a full SystemServer take down.

Change-Id: Ic433d98170470f8cc692ffce4811ab005e265f04