Fix a potential NPE and a case where callsign location source could be reported as None instead of DXCC.

data/callsign.py

@@ -38,7 +38,7 @@ class Callsign:
     # ITU zone in which the callsign indicates they are operating
     itu_zone: int | None = None
     # Location source
-    location_source: LocationSourceForCallsign = LocationSourceForCallsign.NONE
+    location_source: LocationSourceForCallsign | None = None
 
     def fully_populated(self):
         """Utility method to indicate that the callsign data is fully populated. Multiple providers can return data for