periodic_task_name tries to call get on a possibly NoneType object

[KOliver94]

PR #261 introduced a new bug with the following lines:

django-celery-results/django_celery_results/backends/database.py

Lines 43 to 44 in 5f47db6

	properties = getattr(request, 'properties', {})
	periodic_task_name = properties.get('periodic_task_name', None)

The problem is that request object can contain properties with None value. As a result properites will be None instead of an empty dictionary which will cause an exception when the next line tries to call .get on it.

Update: It might be easily fixable with the following change:

properties = getattr(request, 'properties') or {}