BUG: get_variable for postgres never returns a value (#9710)

Because of this, the insert_or_update() method never uses the "INSERT INTO ... ON CONFLICT DO UPDATE SET ..." command, and the logs constantly show errors adding records to the message cache due to "duplicate key".

program/lib/Roundcube/db/pgsql.php

@@ -173,7 +173,10 @@ public function get_variable($varname, $default = null)
             return rcube::get_instance()->config->get('db_' . $varname, $default);
         }
 
-        $this->variables[$varname] = rcube::get_instance()->config->get('db_' . $varname);
+        $cfgval = rcube::get_instance()->config->get('db_' . $varname);
+        if (isset($cfgval)) {
+            return $cfgval;
+        }
 
         if (!isset($this->variables)) {
             $this->variables = [];