discussion topics: fix sorting nil positions

fixes CNVS-12774

test plan:
  * if you don't have legacy discussions with nil positions, run this:

Course.find(1).discussion_topics.where(pinned: true).update_all(position: nil)

  * open the discussions index page
  * move a discussion using the "Move To" gear menu option
  * verify that the discussion is moved
  * reload the page
  * verify that the change persists

Change-Id: Ibb3afc6a876f359cecba360792009b3db1793785
Reviewed-on: https://gerrit.instructure.com/34230
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>