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 {