Links without contents should generate links with the raw URL as the hyperlink text

[charabaruk]

Currently, link objects with a non-nil link property but with nil contents are exported as the link (without schema/type) wrapped in <em> tags. Instead, slimhtml should do the same as the default HTML export backend, and generate a proper anchor tag with the raw link value as the anchor text.

Test case:

[[https://example.com/test.html]]

Current output:

<em>example.com/test.html</em>

Expected output:

<a href="https://example.com/test.html">https://example.com/test.html</a>