api/serializers: Add missing inizialization (W0201)

Fixes an attribute-defined-outside-init (W0201) pylint error.

Signed-off-by: Fabian P. Schmidt <kerel@mailbox.org>

network/api/serializers.py

@@ -116,6 +116,8 @@ class ObservationSerializer(serializers.ModelSerializer):
 
 class NewObservationListSerializer(serializers.ListSerializer):
     """SatNOGS Network New Observation API List Serializer"""
+    transmitters = {}
+
     def validate(self, attrs):
         """Validates data from a list of new observations"""
         station_set = set()