[7.x] Add default value to HtmlString constructor

[mvdnbrk]

This PR adds a default value to the Illuminate\Support\HtmlString constructor.

new HtmlString;

// vs

new HtmlString('');

Example use case:

public function getContent(): HtmlString
{
    if (empty($this->attribute) {
        return new HtmlString;
        // vs
        return new HtmlString('');
    }

    return new HtmlString($this->attribute);
}

[mvdnbrk]

@taylorotwell Thank you!