add validation to prevent blank assignment titles

fixes CNVS-12938

test plan:
  - via api and via the old assignment index
  - try to create an assignment with name of " "
    (there is a space between those quotes)
  - this assignment doesn't save
  - assignments w/o blank names do save

Change-Id: I531f8be005b1c4b8f542cb281f0672669e181fb2
Reviewed-on: https://gerrit.instructure.com/35237
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>