Crocodoc: handle nil response from upload

Test plan:
  submit a Crocodocable assignment.  it should upload successfully (if
  Crocodoc cooperates)

Change-Id: I16f99526bb4a32467a584cc2ab6b6b824e13b358
Reviewed-on: https://gerrit.instructure.com/32523
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>