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_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/45fc3ce22482ec50c0e1b5817782970bf03dbc0b

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