corrected a bug with None tag columns

cdplib/db_handlers/InfluxdbHandler.py

@@ -161,7 +161,7 @@ class InfluxdbHandler:
         """
         
         measurement_columns = [c for c in dataframe.columns
-                               if c not in tag_columns]
+                               if c not in (tag_columns or [])]
         
         for column in measurement_columns:
             try: