Empty elements referenced by aria-labelledby should fail

[WilcoFiers]

The following should result in two failures for the button element:

<button aria-label="foo" aria-labelledby="foo"></button>
<button>
  <img alt="foo" aria-labelledby="foo">
</button>
<div id="foo"></div>

I'm not sure why these elements aren't getting an accessible name in Chrome. I'm pretty sure these used to get an accessible name. We should test if assistive tech also think these are empty. This issue is going to be true for quite a lot of rules too,

Fixing this is going to be tricky. Axe-core runs any checks with these, so right now if a button has aria-label it just passes. We'll have to somehow prevent that if the button also has aria-labelledby? We'll need to think about how to clearly communicate this. This is unintuitive that an empty aria-labelledby ref is not ignored while a missing ref, or an empty aria-label is ignored.