When database is empty, get_or_create_pool_for_session would create a new
pool via metadata_connection_config, which clears the database for Doris
and StarRocks (is_metadata_connection_scoped). The resulting connection
URL had no database path, causing "Current database is not selected"
errors on query execution.
Fall back to the shared pool (get_or_create_pool) when database is empty,
matching the 0.5.19 behavior. Session-scoped pools are still used when
a database is explicitly set.