ui: tape: TapeBackupJob: use correct default value for pbsUserSelector

if we want the empty value as a valid default value in a combogrid,
we have to explicitely select 'null' else the field will be marked as
dirty

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Reported-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

www/tape/window/TapeBackupJob.js

@@ -70,6 +70,7 @@ Ext.define('PBS.TapeManagement.BackupJobEdit', {
 		fieldLabel: gettext('Notify User'),
 		emptyText: 'root@pam',
 		allowBlank: true,
+		value: null,
 		renderer: Ext.String.htmlEncode,
 	    },
 	],