[items] Fix posting/commanding Item with undefined

An error was thrown when undefined was sent to an Item, due to missing
handling of it.

Signed-off-by: Florian Hotze <florianh_dev@icloud.com>

items/items.js

@@ -63,6 +63,7 @@ const DYNAMIC_ITEM_TAG = '_DYNAMIC_';
  */
 function _toOpenhabString (value) {
   if (value === null) return 'NULL';
+  if (value === undefined) return 'UNDEF';
   if (typeof value === 'number' || typeof value === 'string') {
     return value;
   } else if (typeof value.toOpenHabString === 'function') {