Include links with no text as "non-descriptive" in SEO audit

[patrickhulce]

Summary
Split out from #6483 (comment) per @rviscomi 's request. We should consider including all links that don't have any text content in our non-descriptive check. A link that doesn't have any text at all is just as non-descriptive as these other links :)

I think the main concern is that this will start including a bunch of things we don't foresee.

[rviscomi]

Thanks for filing this @patrickhulce! Will investigate.

[rviscomi]

Assigning to Matt to start implementing and testing on popular sites. Let's see what the new failure rate is before releasing it.

[mattzeunert]

Ran on 100 sites from here.

	Before	After
Failure rate	38%	81%
Average item count	0.75	10.5
Median item count	0	3

Right now the AnchorElements artifact uses innerText as the link text, which means the text is empty if the link is hidden. I've used textContent for now. If we don't want to count invisible elements we can detect that in the gatherer.