Handle concurrent sequence drops during synchronization

Commit d4a657b0a4d added a call to has_sequence_privilege() while
fetching sequence information from the publisher, so that
publisher-side permission failures could be distinguished from missing
sequences. It also assumed that has_sequence_privilege() could never
return NULL, and asserted accordingly.

However, that assumption was incorrect. If a sequence is dropped after
the synchronization worker collects its metadata but while fetching the
sequence information, has_sequence_privilege() can return NULL.
This can trigger the assertion failure. This was also reported in
a buildfarm failure on member culicidae.

Fix this by treating a NULL result from has_sequence_privilege() as
indicating that the sequence was dropped concurrently, and report it as
a missing sequence instead of asserting that the result is never NULL.

Reported-by: Noah Misch <[email protected]>
Author: Vignesh C <[email protected]>
Reviewed-by: Hayato Kuroda <[email protected]>
Reviewed-by: Zsolt Parragi <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Discussion: 
https://postgr.es/m/caldanm2fhgleiqkj0r6og7n9qeayxsmplrwyjryt4dl_m3v...@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/55f518684895bfe43e667acf59284b49707ee685

Modified Files
--------------
src/backend/replication/logical/sequencesync.c | 15 +++++--
src/test/subscription/t/036_sequences.pl       | 56 +++++++++++++++++++++++++-
2 files changed, 67 insertions(+), 4 deletions(-)

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