ui: improve render_optional_namespace slighly

it maybe should still simple get dropped and replaced with
(empty)Text 'Root' or 'Root Namespace'

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

www/Utils.js

@@ -686,7 +686,7 @@ Ext.define('PBS.Utils', {
     },
 
     render_optional_namespace: function(value, metadata, record) {
-	if (!value) return '-'; // FIXME ??
+	if (!value) return `- (${gettext('Root')})`;
 	return Ext.String.htmlEncode(value);
     },