Backup generation fails if there is a gravity log with an unlinked device

[Deleted user]

This can happen, as the device is set null on gravity sensor deletion

AttributeError: 'NoneType' object has no attribute 'uuid'
(11 additional frame(s) were not displayed)
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  File "backups/models.py", line 95, in generate_backup_json
    backup_dict = cls.generate_backup_dict()
  File "backups/models.py", line 83, in generate_backup_dict
    'gravity_logs': backup_funcs.dump_gravity_logs(),
  File "backups/backup_funcs.py", line 32, in dump_gravity_logs
    return [log.to_dict() for log in GravityLog.objects.all()]
  File "backups/backup_funcs.py", line 32, in <listcomp>
    return [log.to_dict() for log in GravityLog.objects.all()]
  File "gravity/models.py", line 368, in to_dict
    'device_uuid': str(self.device.uuid),