closes FOO-2110
flag=none
TEST PLAN:
1) try to create a feature flag log record with the user id -1
2) it should fail the reference check and refuse.
3) "null" users should still be allowed (so jobs can
change feature state if need be)
Change-Id: Ib9bbc8a3f141471858ca66d992dc761f2396fc4c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/267687
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>