Fails when using ModelPusherMixin with only get_queryset implemented

[mattjegan]

When a viewset only has get_queryset implemented but fails to declare a queryset class property, DRF model pusher errors when trying to find the backend. I think it should try it's current way but if no queryset exists on the view, call get_queryset or even allow specific backends to be declared on the view.

  File "/Users/abc/Documents/xyz/Code/abc-api/.venv/lib/python3.6/site-packages/rest_framework/viewsets.py", line 83, in view
    self = cls(**initkwargs)
  File "/Users/abc/Documents/xyz/Code/abc-api/.venv/lib/python3.6/site-packages/drf_model_pusher/views.py", line 26, in __init__
    self.pusher_backends = self.get_models_pusher_backends()
  File "/Users/abc/Documents/xyz/Code/abc-api/.venv/lib/python3.6/site-packages/drf_model_pusher/views.py", line 30, in get_models_pusher_backends
    model = self.queryset.model
AttributeError: 'NoneType' object has no attribute 'model'