fill null for tractate

src/models/esmm/fe/tractate_fe.py

@@ -413,6 +413,7 @@ def tractate_feature_engineering(tractate_df):
 
     df["service_id"] = df["service_city"].fillna("-1")
     df["recommend_service_id"] = df["recommend_service_id"].fillna("-1")
+    df["recommend_service_city"] = df["recommend_service_city"].fillna("")
 
     df = df[TRACTATE_COLUMNS]