MDL-70965 core_dml: php8 related changes:

1. pg_connection_status(false) fails on PHP8 because the argument is not a stream
2. skipeed the unittest that expected that pg_query() does not show an error if the resource is invalid
see also MDL-71482

lib/dml/pgsql_native_moodle_database.php

@@ -192,7 +192,7 @@ class pgsql_native_moodle_database extends moodle_database {
         $dberr = ob_get_contents();
         ob_end_clean();
 
-        $status = pg_connection_status($this->pgsql);
+        $status = $this->pgsql ? pg_connection_status($this->pgsql) : false;
 
         if ($status === false or $status === PGSQL_CONNECTION_BAD) {
             $this->pgsql = null;