"td-has-header" or no other rule fails when the 'headers' attribute on a <th> element is empty

```td-has-header``` fails when `````` element has ```headers=""``` but not when `````` element has ```headers=""```

Expectation: Flag `````` elements with ```headers=""```

Actual: `````` elements with ```headers=""``` are not flagged

Motivation: There is a chance an author might have missed pointing the `````` (i.e. sub-heading) element to the parent header

Repro example:
```


Homework
Exams
Projects


1
2
Final
1
2
Final


15%
15%
15%
20%
10%
10%
15%

```

axe-core version: 3.4.1-canary.6e4ed6b

axe-Coconut version: v4.2.1