Handle concurrent sequence drops during synchronization

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 <noah@leadboat.com>
Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com
Discussion: https://postgr.es/m/CALDaNm2fHGLeiQKj0r6OG7N9QeayxSmpLrWYJRyt4dL_m3VRWw@mail.gmail.com
Backpatch-through: 19