conversations: handle ghost conversations

fixes CNVS-11205

I don't know how we're still getting empty conversations
in people's inboxes, but we can't let it break everything.
See https://gerrit.instructure.com/#/c/27121/
and https://gerrit.instructure.com/#/c/26059/ .

test plan:
  * In your Rails console, run something like this:
    user = User.where(name: 'banderson@instructure.com').first
    deleted = user.all_conversations.where(message_count: 0).limit(1)
    deleted.update_all(last_message_at: Time.now)
  * Open your user's inbox in old conversations.
  * Click on each of the conversations in the inbox and verify that the
      corresponding messages load in the right pane.

Change-Id: Ia605ac0b2392ca4b1983bca291d4685a5a818f7a
Reviewed-on: https://gerrit.instructure.com/30159
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>