Add --if-exists to pg_dump command-line in backup creation

Because we use the --clean argument to `pg_dump`, the database backup
SQL file has many DROP commands that precede the section to restore the
tables and data. However, on instances with an empty database, these
DROP statements generate a lot of noise and make it difficult to filter
true error messages from noise.

In preparation of capturing error messages from stderr, we add this to
the backup commands to make it easier to flag real errors.

Reference: https://www.postgresql.org/docs/11/app-pgdump.html

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/36405

changelogs/unreleased/sh-add-if-exists-pgdump.yml

+---
+title: Add --if-exists to pg_dump command-line in backup creation
+merge_request: 40792
+author:
+type: other

lib/backup/database.rb

@@ -27,6 +27,7 @@ module Backup
           progress.print "Dumping PostgreSQL database #{config['database']} ... "
           pg_env
           pgsql_args = ["--clean"] # Pass '--clean' to include 'DROP TABLE' statements in the DB dump.
+          pgsql_args << '--if-exists'
 
           if Gitlab.config.backup.pg_schema
             pgsql_args << '-n'