pg_upgrade: Fix detection of invalid logical replication slots.

pg_upgrade: Fix detection of invalid logical replication slots.

Commit 7a1f0f8747a optimized the slot verification query but
overlooked cases where all logical replication slots are already
invalidated. In this scenario, the CTE returns no rows, causing the
main query (which used a cross join) to return an empty result even
when invalid slots exist.

This commit fixes this by using a LEFT JOIN with the CTE, ensuring
that slots are properly reported even if the CTE returns no rows.

Author: Lakshmi N <lakshmin.jhs@gmail.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CA+3i_M8eT6j8_cBHkYykV-SXCxbmAxpVSKptjDVq+MFtpT-Paw@mail.gmail.com