fix(dbapi): avoid running % format when no query parameters are passed

[tswast]

Is your feature request related to a problem? Please describe.

It is unexpected to get format errors when a string contains %, but there are no query parameters in the query.

googleapis/python-bigquery-sqlalchemy#72

Describe the solution you'd like

In addition to checking if parameters is none, check if len(parameters) == 0 to avoid unnecessary format operations.

python-bigquery/google/cloud/bigquery/dbapi/cursor.py

Line 444 in dca2e4c

if parameters is None: