rails3: fix discussion topic integration spec

rails 3 raises an exception when passing a nil user to
group_user_url, while rails 2 returned a url with an incorrect
user id.

test plan:
 - create a discussion topic without an author, or remove the
 author from an existing discussion.
 - open the discussion in the browser and make sure it displays
 like it used to.

Change-Id: Ic7973cd5eb35c5f6b428e9a2edfe1b23f64ecf63
Reviewed-on: https://gerrit.instructure.com/30363
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>