handle percent assignment with nil points possible

fixes CNVS-16374

test plan:
- create a percent grading assignment and leave the points possible field blank
- give someone a grade
- it should work (although it will just turn your grade directly into
  a percentage, since mathematically this doesn't make sense)

Change-Id: I48cfbf8ee2be113f3995aa91461a77b062485cdf
Reviewed-on: https://gerrit.instructure.com/43137
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>