pgsql: Avoid harmless warning from pg_dump --if-exists mode.

Avoid harmless warning from pg_dump --if-exists mode.

If the public schema has a non-default owner (perhaps due to
dropping and recreating it) then use of pg_dump's "--if-exists"
option results in a warning message:

warning: could not find where to insert IF EXISTS in statement "-- *not* dropping schema, since initdb creates it"

This is harmless since the dump output is the same either way,
but nonetheless it's undesirable. It's the fault of commit
a7a7be1f2, which created situations where a TOC entry's "defn"
or "dropStmt" fields could be just comments. Although that
commit fixed up the kluges in pg_backup_archiver.c that munge defn
strings, it missed doing so for the one that munges dropStmts.

Per bug# 17753 from Justin Zhang.

Discussion: https://postgr.es/m/17753-9c8773631747ee1c@postgresql.org

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/488e89bf725a3b4a1caf9e3e82ae6e75e914d123

Modified Files
--------------
src/bin/pg_dump/pg_backup_archiver.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)