vacuumdb: Fix --missing-stats-only with virtual generated columns.

vacuumdb: Fix --missing-stats-only with virtual generated columns.

Statistics aren't created for virtual generated columns, so
"vacuumdb --missing-stats-only" always chooses to analyze tables
that have them.  To fix, modify vacuumdb's query for retrieving
relations that are missing statistics to exclude those columns.

Oversight in commit edba754f05.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Discussion: https://postgr.es/m/20250820104226.8ba51e43164cd590b863ce41%40sraoss.co.jp
Backpatch-through: 18