Don't fail on malformed POST query

[mpasternak]

Stacktrace (most recent call last):

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...
  File "bpp/views/mymultiseek.py", line 18, in get_queryset
    qset = super(MyMultiseekResults, self).get_queryset()
  File "multiseek/views.py", line 282, in get_queryset
    self.request.session.get(MULTISEEK_SESSION_KEY_REMOVED, []))
  File "multiseek/logic.py", line 674, in get_query_for_model
    retval = self.model.objects.filter(query)
ValueError: Cannot use None as a query value