a11y: default listing emits empty <a> when a post has no subtitle (link-name)

[cderv]

Reported in #14373.

Repro: https://github.com/alexpghayes/quarto-accessibility-reprex

axe rule: link-name · impact: serious · tag: wcag2a (2.4.4)
Target: .listing-subtitle > a.no-external — rendered as <a href="..." class="no-external" data-original-href="..."></a> (empty)

The default listing template renders an anchor inside .listing-subtitle for every post even when the post has no subtitle field. The anchor is empty (no text, no aria-label, no title), ends up in the tab order, and fails link-name. The template should skip the subtitle wrapper when no subtitle is present, or render it without a focusable anchor.

Different element than #7088 (which was a listing body-level empty <a>, fixed in v1.4).

Part of #8706.