Reverse logic of html5() Method

Resolves issue where calling $qp('<strong>Content</strong>')->html5() would return an empty string, and calling $qp('<strong>Content</strong>')->html5('') would return <strong>Content</strong>. The correct logic, per ->html(), is to reverse how this method works.

src/DOMQuery.php

@@ -718,7 +718,7 @@ public function html5($markup = null)
  $html5 = new HTML5($this->options);
 
  // append HTML to existing
- if ($markup === null) {
+ if (isset($markup)) {
  // Parse the HTML and insert it into the DOM
  $doc = $html5->loadHTMLFragment($markup);
  $this->removeChildren();