fix(assistance): Category can be empty for Incident Ticket

ref: #537 #577

app/itim/serializers/incident.py

@@ -28,6 +28,7 @@ class IncidentTicketModelSerializer(
 ):
 
     category = serializers.PrimaryKeyRelatedField(
+        allow_null = True,
         queryset = TicketCategory.objects.filter(
             incident = True
         ),