Fix concurrent sequence drops during sequence synchronization.

Fix concurrent sequence drops during sequence synchronization.

A recent BF failure showed that commit 7a485bd641 did not handle the case
where a sequence is dropped concurrently during sequence synchronization
on the subscriber. Previously, pg_get_sequence_data() would ERROR out if
the sequence was dropped concurrently. After 7a485bd641, it instead
returns NULL, which leads to an assertion failure on the subscriber.

To handle this change, update sequence synchronization to skip sequences
for which pg_get_sequence_data() returns NULL.

Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CALDaNm0FoGdt+1mzua0t-=wYdup5_zmFrvfNf-L=MGBnj9HAcg@mail.gmail.com