Avoid permission failure in pg_sequences.last_value

Before, reading pg_sequences.last_value would fail unless the user had
appropriate sequence permissions, which would make the pg_sequences view
cumbersome to use.  Instead, return null instead of the real value when
there are no permissions.

From: Michael Paquier <michael.paquier@gmail.com>
Reported-by: Shinoda, Noriyoshi <noriyoshi.shinoda@hpe.com>