github sync: empty link html contents cannot be parsed

[yeap-oneke]

Bug description

When sync documents from github, if the documents contain links with empty href, then the link and the contents is lost in gitbook.

<b>use</b> <a href="">0x1::error</a>;
<b>use</b> <a href="">0x1::event</a>;
<b>use</b> <a href="">0x1::object</a>;
<b>use</b> <a href="">0x1::option</a>;
<b>use</b> <a href="">0x1::signer</a>;

Additional context