Fix - Optimisation when min_cardinality is not set
Fix - Optimisation when min_cardinality is not set

We do not filter by cardinality if it is not needed (as
expected). min_cardinality was set to 0 is it was 'None'
so the condition was never True.

Pierre-Sassoulas/django-survey,Pierre-Sassoulas/django-survey,Pierre-Sassoulas/django-survey