fix response type for invoice patch

[diego-escobedo]

Resolves LOT-687

[linear]

LOT-687 Fix Mark Invoice as Paid functionality

Sentry Issue: PROD-WK

AssertionError: Expected a `Response`, `HttpResponse` or `HttpStreamingResponse` to be returned from the view, but received a `<class 'NoneType'>`
  File "django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "rest_framework/viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
  File "api/views.py", line 1566, in dispatch
    response = super().dispatch(request, *args, **kwargs)
  File "rest_framework/views.py", line 511, in dispatch
    self.response = self.finalize_response(request, response, *args, **kwargs)
  File "rest_framework/views.py", line 423, in finalize_response
    assert isinstance(response, HttpResponseBase), (