fix: don't use a list for empty arguments (#750)

Author: IlyaFaer

google/cloud/spanner_dbapi/parse_utils.py

@@ -233,7 +233,7 @@ def sql_pyformat_args_to_spanner(sql, params):
  arguments.
  """
  if not params:
- return sanitize_literals_for_upload(sql), params
+ return sanitize_literals_for_upload(sql), None
 
  found_pyformat_placeholders = RE_PYFORMAT.findall(sql)
  params_is_dict = isinstance(params, dict)