pgsql: Skip relations dropped concurrently in GetSubscriptionRelations(

Skip relations dropped concurrently in GetSubscriptionRelations().

GetSubscriptionRelations() can see a pg_subscription_rel row for a
relation whose pg_class row was removed by a concurrent DROP. In this
case, get_rel_relkind() returns '\0'. Skip such relations since they no
longer exist and do not need synchronization.

Replace the assertion on relkind with an error. Once the dropped
relation case is handled, any other unexpected relkind indicates a
relation kind that cannot be part of a subscription and should be
reported rather than ignored.

Author: Vignesh C <vignesh21(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Backpatch-through: 19
Discussion: https://postgr.es/m/CALDaNm3eeKocRtQyUPdg2kaxJ-VAo5pwcNytDKNT1Yc0t2V_ug%40mail.gmail.com

Branch
------
REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4680f32d333da51db3034047595dc0f567410625

Modified Files
--------------
src/backend/catalog/pg_subscription.c | 31 ++++++++++++++++++++++---------
1 file changed, 22 insertions(+), 9 deletions(-)