pg_createsubscriber: Add log message when no publications exist to drop.

When specifying --clean=publication to pg_createsubscriber, it drops
all existing publications with a log message "dropping all existing
publications in database "testdb"". Add a new log message "no
publications found" when there are no publications to drop, making the
progress more transparent to users.

Author: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAHut+Ptm+WJwbbYXhC0s6FP_98KzZCR=5CPu8F8N5uV8P7BpqA@mail.gmail.com

src/bin/pg_basebackup/pg_createsubscriber.c

@@ -1755,10 +1755,15 @@ check_and_drop_publications(PGconn *conn, struct LogicalRepInfo *dbinfo)
 			disconnect_database(conn, true);
 		}
 
-		/* Drop each publication */
-		for (int i = 0; i < PQntuples(res); i++)
-			drop_publication(conn, PQgetvalue(res, i, 0), dbinfo->dbname,
-							 &dbinfo->made_publication);
+		if (PQntuples(res) > 0)
+		{
+			/* Drop each publication */
+			for (int i = 0; i < PQntuples(res); i++)
+				drop_publication(conn, PQgetvalue(res, i, 0), dbinfo->dbname,
+								 &dbinfo->made_publication);
+		}
+		else
+			pg_log_info("no publications found");
 
 		PQclear(res);
 	}