Autograding fails if assignment properties is None

[meffmadd]

Describe the bug
_write_gradebook does not accept a gradebook string that is None and fails. This happens when the assignment was not pushed beforehand.

To Reproduce
Steps to reproduce the behavior:

1. Delete database
2. Create (new) assignment (it has null as the properties by default)
3. Create a submission and auto-grade it
4. See error

Expected behavior
An error message or a meaningful error message in the logs...