Change root item check to test against null and undefined

Author: philipstanislaus

src/arrayToTree.ts

@@ -58,7 +58,7 @@ export function arrayToTree (items: Item[], config: Partial<Config> = {}): TreeI
 
     const TreeItem = lookup[itemId]
 
-    if (!parentId) {
+    if (parentId === null || parentId === undefined) {
       // is a root item
       rootItems.push(TreeItem)
     } else {