Fix closing tags for bricks with no content

The second parameter to the `Html::tag` function (which the `Brick` class uses internally) should be `null` if the tag has no content, like an `<img>` or `<hr>` tag for example.

`$brick -> html()` currently returns `false` instead of `null` if the content is empty, which leads to the element being closed explicitly.

Example from the documentation:

```php
$img = brick('img');
$img->attr('src', 'http://yourdomain.com/someimage.jpg');
$img->attr('width', 300);
$img->attr('height', 200);
$img->addClass('image');

echo $img;
```

This currently prints

```html
<img src="http://yourdomain.com/someimage.jpg" width="300" height="200" class="image"></img>
```

but should print

```
<img src="http://yourdomain.com/someimage.jpg" width="300" height="200" class="image" />
```

This PR changes the `Brick` initializer and helper defaults to use `null` instead.