pgsql: Don't emit garbage "\[un]restrict (null)" commands in pg_dump.

Don't emit garbage "\[un]restrict (null)" commands in pg_dump.

_reconnectToDB() failed to consider the possibility that it could
be called without any restrict_key set. This is reachable while
generating the "restore.sql" file in tar dump mode, which nobody
noticed because that file is mostly unused (and nothing really
bad happens anyway unless you specified --inserts mode). But
I wonder if there are other code paths that reach the issue.

Bug: #19718
Reported-by: Pavel Kopylov <pkopylov(at)cloudlinux(dot)com>
Author: Pavel Kopylov <pkopylov(at)cloudlinux(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/19718-7e945d0ff9b5a589@postgresql.org
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ae63671a7fb1c297c39027e0b29e6bed8d8aa5e2

Modified Files
--------------
src/bin/pg_dump/pg_backup_archiver.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)