fix student reports with no params in api

fixes CNVS-11479

test plan
 - in api with no params do
 - run zero activity report
 - run no submissions report
 - reports should not fail

Change-Id: I059be8dfaab2658f4c7008acde92a971e9e0208b
Reviewed-on: https://gerrit.instructure.com/30896
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>