corrected a bug with None tag columns

cdplib/db_handlers/InfluxdbHandler.py

@@ -166,7 +166,7 @@ class InfluxdbHandler:
         for column in measurement_columns:
             try:
                 self.client.write_points(
-                    dataframe=dataframe[[column] + tag_columns],
+                    dataframe=dataframe[[column] + (tag_columns or [])],
                     measurement=column,
                     tag_columns=tag_columns,
                     protocol='line',