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

core/sig_lookup_helper.py

@@ -43,8 +43,9 @@ def get_sig_ref_info(sig_name, ref_id):
         # database.
         if sig_name.upper() == "DME":
             match = re.match(r"DME[\- ](\d{3,5})", ref_id, re.IGNORECASE)
-            number = match.group(1)
-            ref_id = f"DME-{number.zfill(5)}"
+            if match:
+                number = match.group(1)
+                ref_id = f"DME-{number.zfill(5)}"
 
         # DTMBA spotters sometimes include spaces and dashes, our regex allows them but they must be removed here so we
         # can look up against the official list which doesn't have them