Asyncpg null query fix

Asyncpg null query fix

Adjusted the asyncpg dialect so that an empty SQL string, which is valid
for PostgreSQL server, may be successfully processed at the dialect level,
such as when using :meth:`.Connection.exec_driver_sql`. Pull request
courtesy Andrew Jackson.

Closes: #12220
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12220
Pull-request-sha: 45c94febee66b567040b1fbfa3a93079a1314f09

Change-Id: I870df9e31f4a229939e76c702724c25073329282
(cherry picked from commit 7bfb829f25c1bfe2139afe7875882298aaf345ba)