a11y: Fix empty focusable link in reference Jump To nav

[coseeian]

This PR removes an empty, invisible, but keyboard-focusable link from the reference sidebar reported by a11y page tests. This fixes a WCAG 2.0 Level A failure.

The Problem

Some reference entries carry empty submodule information. As a result, the jump to submodule link in ReferenceItemLayout was generating empty anchors:

<li class="_linklabel_1jot8_995"><a href="/reference/#"></a></li>

Because an anchor without text collapses to 0x0 in size, it is visually invisible and unclickable. However, it remains in the DOM tab order. Keyboard and screen reader users tabbing through the sidebar encounter a ghost focus stop that announces as "link" only.

The Fix

• Conditional rendering: The jump-to-submodule link is now only rendered when a valid submodule exists.
• Safeguard in JumpToLinks: Added a check so JumpToLinks skips any label-less links, preventing empty anchors from returning via another collection's Jump To state.