Fix slim/html5 templates to not render empty style tag

**Before:**
```html
<div class="imageblock" style="">
  <div class="content">
    <img alt="sunset" src="sunset.jpg">
  </div>
</div>
```

**After:**
```html
<div class="imageblock">
  <div class="content">
    <img alt="sunset" src="sunset.jpg">
  </div>
</div>
```
Yeah, #79.