prevents Post Grades button from appearing without assignments

fixes SIS-652

Test plan:
- create a new course and publish it
- add some users
- go to the gradebook
- confirm the 'Post Grades' button is gone
- create and assignment with the 'Post to SIS' option selected
- publish the assignment
- go to the gradebook
- confirm the 'Post Grades' button is there and works properly

Change-Id: Ie1f5e44a31b77402f1a102d10a9d7e8df6feec11
Reviewed-on: https://gerrit.instructure.com/45333
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>