Widget expressions: (Partly) Fix handling of Item name being `undefined` (#2301)

When using the `@` and `@@` shortcuts for Item displayState and state in
widget expressions, properly handle cases where the Item name is
`undefined`.
Unfortunately I did not find a way to improve that for `items[itemName]`
expressions.

Requesting the Item state from the server when the Item name is
undefined leads to warnings in the log (which might cause some confusion
for the users):
```
[WARN ] [se.internal.SseItemStatesEventBuilder] - Attempting to send a state update of an item which doesn't exist: undefined
```

---------

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

bundles/org.openhab.ui/web/src/components/widgets/widget-mixin.js

@@ -19,11 +19,12 @@ import jsepTemplate from '@jsep-plugin/template'
 expr.jsep.plugins.register(jsepRegex, jsepArrow, jsepObject, jsepTemplate)
 
 expr.addUnaryOp('@', (itemName) => {
+  if (itemName === undefined) return undefined
   const itemState = store.getters.trackedItems[itemName]
-  if (itemState.displayState === undefined) return itemState.state
-  return itemState.displayState
+  return itemState.displayState || itemState.state
 })
 expr.addUnaryOp('@@', (itemName) => {
+  if (itemName === undefined) return undefined
   return store.getters.trackedItems[itemName].state
 })