fix #4951: accept undefined as value for the MultiDiskSelector

otherwise it tries to string split it and throws an exception

This can happen when there was no initial value and the form is reset.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/form/MultiDiskSelector.js

@@ -39,6 +39,8 @@ Ext.define('Proxmox.form.MultiDiskSelector', {
     setValue: function(value) {
 	let me = this;
 
+	value ??= [];
+
 	if (!Ext.isArray(value)) {
 	    value = value.split(/;, /);
 	}