Modifies subscription_hold to return nil if user is nil

fixes CNVS-15356

Test Plan:
  - Add a discussion to a course that has the following options:
    * Group discussion
    * Allow threaded replies
    * Graded
    * Set a due date of sometime in the future
  - Make the course syllabus publicly available
  - In another window (incognito) go to the syllabus
  - The syllabus should load without problems.

Change-Id: I177555c6e0ac87554360361a95b1ff0a26d35883
Reviewed-on: https://gerrit.instructure.com/41176
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>