Do not display HTML links when current search has no results and adding another filter wouldn't add more.

[isoos]

• Similarly to Hide sort control when there is no result to be sorted. #8661, reduces the link surface, so indiscriminate crawlers have fewer things to request from us.
• The complexity comes from is:unlisted, as it is not restricting, rather expanding the result set, so we need to pass the appropriate information down as well.
• The web app's dynamic update is also adjusted to work with the <span> as well as the <a> element.

[isoos]

Note: pushed to staging for further testing.

[isoos]

Offline discussion prompted to put a bit more effort into the index before removing these links, closing for now (#8670, #8671).