Treat an empty string from the admin form to be None/NULL for the Course...

...EmailTemplateForm
Rather than relying on `get` throwing an exception to fail the test, it would be better if you used an actual assertion:
```python
cet = CourseEmailTemplate.objects.get(name=None)
self.assertIsNotNone(cet)
```
Please apply this to the other tests in this PR, as well.