Added exception handling on internal connection_fetch_live_columns about the config_json_string, which is optional on the Public form of the API

Author: bryanthowell-ts

src/thoughtspot_rest_api_v1/tsrestapiv1.py

@@ -1863,6 +1863,8 @@ def connection_fetch_live_columns(self, connection_guid, database_name: str,
 
         if use_internal_endpoint is True:
             url = self.non_public_base_url + endpoint
+            if config_json_string is None:
+                raise Exception('The config_json_string (a JSON object converted to string using json.dumps() ) is required')
         else:
             url = self.base_url + endpoint
         tables = [{"databaseName": database_name,