Prevent TypeError when task json data missing

[alanhamlett]

Related to #112.

This prevents an exception when calling Task.tasks_from_queue, but this doesn't fix the root problem of the task being created with invalid json data:

tasktiger/tasktiger/task.py

Line 312 in 10b5fe7

serialized_task = json.dumps(self._data)

[thomasst]

this doesn't fix the root problem of the task being created with invalid json data:

I've seen this sometimes, but usually only if I manually messed with Redis to clear keys and didn't properly clean up, in which case the key doesn't exist.

I thought of making this change as well, but I think I'd prefer us to fail explicitly here.

Any further thoughts @jkemp101 ?