Only return series where relays have been reported

opensensor/collection_apis.py

@@ -402,6 +402,7 @@ def get_relay_board_pipeline(
     if end_date is None:
         end_date = datetime.utcnow()
     match_clause = get_initial_match_clause(device_ids, device_name, start_date, end_date)
+    match_clause["relays"] = {"$exists": True}
 
     # The MongoDB aggregation pipeline for Relay Board data
     pipeline = [