Sidebar header navigation renders empty anchor when heading is a Markdown link

Problem

A heading is a Markdown link causes the sidebar header navigation item to render as an empty anchor.

For example, a heading like:

## [fzf](https://github.com/junegunn/fzf)

can produce sidebar HTML like:

<li class="header-item expanded">
  <span class="chapter-link-wrapper">
    <a href="#fzf" class="header-in-summary current-header"></a>
  </span>
</li>

The <a> is present, but it has no text content, so the sidebar entry appears blank.

The sidebar generates a matching anchor element for the heading, but the rendered text content is empty.

See https://sigoden.github.io/mynotes/collections/cli.html for an example.

Steps

1. run mdbook init
2. modify src/chapter_1.md

# Chapter 1

## [fzf](https://github.com/junegunn/fzf)

An interactive filter program for any kind of list.

## [rg](https://github.com/BurntSushi/ripgrep)

Fast line-oriented search tool.

3. run mdbook serve

Possible Solution(s)

The sidebar header navigation should display the heading text, even if the heading is a Markdown link.

At minimum, headings with links should not produce an empty sidebar entry.

Notes

No response

Version

mdbook v0.5.3

Reproduce the issue with mdbook init, the sample heading in src/chapter_1.md, and mdbook serve. Start by inspecting the generated sidebar entry for the Markdown-linked heading; done when the sidebar displays the heading text instead of an empty anchor.