Removes attributes which have a value of undefined.
```html
<div id="undefined"></div>
<!-- is now -->
<div></div>
```
```html
<h1>Hello undefined!</h1>
<!-- is now -->
<h1>Hello !</h1>
```
This may be considered backwards incompatable and may need to wait until Svelte 3. If existing apps rely on e.g. querySelector('[id="undefined"]') then this could require changes.