avoid writing bogus attributes during report

refs CNVS-15830

test plan:
 * run the student_assignment_outcome_map report
 * it shouldn't asplode, and assignment url and submission date
   columns should have data (once you have any rows in the report
   at all - looks like you need a submission for an assignment tied
   to an outcome)

Change-Id: I0a202dc9c08ee8a95d614105f31a13f0cf961a0c
Reviewed-on: https://gerrit.instructure.com/41981
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>