Don't emit empty alt tag if image description is empty.

An empty but present alt attribute indicates to browsers that the
image is not a key part of the content.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Img

Closes #718.

spec.txt

@@ -8603,7 +8603,7 @@ My ![foo bar](/path/to/train.jpg  "title"   )
 ```````````````````````````````` example
 ![](/url)
 .
-<p><img src="/url" alt="" /></p>
+<p><img src="/url" /></p>
 ````````````````````````````````