Only add img[alt] when [data-alt] exists

Fix a bug where, when the element specified by [data-picture] did not have
a [data-alt] attribute, the generated image element would have an [alt]
attribute set to the string value "null".

<span data-picture> => <img alt="null" src="...">

Now, the following hold:

<span data-picture> => <img src="...">
<span data-picture data-alt=""> => <img alt="" src="...">
<span data-picture data-alt="alt text"> => <img alt="alt text" src="...">
Merge pull request #158 from bryanburgers/alt-bug

Only add img[alt] when [data-alt] exists