fixes bad enrollments.user_id query

fixes CNVS-10572

test plan
 - should generate an error report when this
   happens instead of a database error
 - specs should pass

Change-Id: I599cd420fbdd4f417f64252d259858df4a2969c1
Reviewed-on: https://gerrit.instructure.com/33194
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>