[refer #PGPRO-3213] Use empty string as default value of OnlineUpgradePath instead of NULL

Author: knizhnik

src/backend/access/transam/xlog.c

@@ -8838,7 +8838,7 @@ CreateCheckPoint(int flags)
 	 * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
 	 * = end of actual checkpoint record.
 	 */
-	if (shutdown && checkPoint.redo != ProcLastRecPtr && OnlineUpgradePath == NULL)
+	if (shutdown && checkPoint.redo != ProcLastRecPtr && *OnlineUpgradePath == '\0')
 		ereport(PANIC,
 				(errmsg("concurrent write-ahead log activity while database system is shutting down")));
 

src/backend/utils/misc/guc.c

@@ -3678,7 +3678,7 @@ static struct config_string ConfigureNamesString[] =
 		 GUC_SUPERUSER_ONLY
 		},
 		&OnlineUpgradePath,
-		NULL,
+		"",
 		check_online_update_support, NULL, NULL
 	},