don't fail sis_imports GET with nil options

fixes CNVS-16235

test plan
 - create an sis_import
 - update the import to have nil options
   s = SisBatch.where(id: last_import)
   s.options = nil
   s.save!
 - go to /api/v1/accounts/self/sis_imports
 - it should not fail

Change-Id: I230f5535726eada520b4369bd01c0e0262e94630
Reviewed-on: https://gerrit.instructure.com/42650
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>