role=tree without children triggering axe error

Using ace 1.3.7. with, e.g.

```xml

tree without children test

```

triggers an aria-required-children error in axe ("Required ARIA children role not present: group, treeitem" etc). However, that's no longer a requirement in ARIA 1.3.

(Using a recent version, axe itself does not seem to complain about this either so perhaps ace's axe fork just needs an update.)