Support rendering of images with an empty alt

[jeisses]

Hi, first of all thanks for creating this great library! I love the no dependency approach.

This is a small fix that supports rendering links and images without an alt. With this change, the following will work:

(clarktown/render "![](https://example.com)")
"<p><img src=\"https://example.com\" alt=\"\"></p>"

I believe that to be the expected behavior of a markdown parser.

[askonomm]

Looks great! Thanks for the contribution 🙌