fix(VTreeView): null check when building tree from children (#13913)

* This fixes children is undefined/null when doing "children.map" when given an undefined/null children value

* Changed the invalid check

* Changed back to a null coalesce operator

* Update packages/vuetify/src/components/VTreeview/VTreeview.ts

Co-authored-by: Jacek Karczmarczyk <[email protected]>

Author: JasonLandbridge

packages/vuetify/src/components/VTreeview/VTreeview.ts

@@ -201,7 +201,7 @@ export default mixins(
       for (let i = 0; i < items.length; i++) {
         const item = items[i]
         const key = getObjectValueByPath(item, this.itemKey)
-        const children = getObjectValueByPath(item, this.itemChildren, [])
+        const children = getObjectValueByPath(item, this.itemChildren) ?? []
         const oldNode = this.nodes.hasOwnProperty(key) ? this.nodes[key] : {
           isSelected: false, isIndeterminate: false, isActive: false, isOpen: false, vnode: null,
         } as NodeState