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 had 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="...">
```