aria-prohibited-attr should always report incomplete if there is text in the subtree

The following should have been reported as incomplete, but is failed instead:

```html
<div role="code" aria-label="foo">Hello world</div>
```

It should only fail if the content is missing, like in the following:

```html
<div role="code" aria-label="foo"></div>
```

This already works without the role. It goes bad because `subtreeText` isn't called correctly.