pgsql: Don't allow to set replication slot_name as ''.

Don't allow to set replication slot_name as ''.

We don't allow to create replication slot_name as an empty string ('') via
SQL API pg_create_logical_replication_slot() but it is allowed to be set
via Alter Subscription command. This will lead to apply worker repeatedly
keep trying to stream data via slot_name '' and the user is not allowed to
create the slot with that name.

Author: Japin Li
Reviewed-By: Ranier Vilela, Amit Kapila
Backpatch-through: 10, where it was introduced
Discussion: https://postgr.es/m/MEYP282MB1669CBD98E721C77CA696499B61A9@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/eb158e74afafd052339729efd179dbf74ecd9683

Modified Files
--------------
src/backend/commands/subscriptioncmds.c    | 3 +++
src/test/regress/expected/subscription.out | 3 +++
src/test/regress/sql/subscription.sql      | 3 +++
3 files changed, 9 insertions(+)