Fix zone filter if no zone is specified in params

backend/right_tree/api/filters.py

@@ -38,9 +38,13 @@ def soil_variant_filter(request, queryset):
 
 def zone_filter(request, queryset):
     zone = request.query_params.get('zone')
+
+    if zone != None:
         zone_json = json.loads(zone)
         return queryset.filter(zones__id__contains=zone_json['id']).distinct()
 
+    return queryset
+
 
 def soil_order_coordinate_filter(coordinates):
     pnt = get_point_from_coordinates(coordinates)