Fix xmldom compatibility (#794)

**Public API Changes**
Extend element typing

**Description**
Element can also be null since `@xmldom/[email protected]` (See xmldom/xmldom#721) 

Fixes #778

Author: MatthijsBurgh

src/urdf/UrdfModel.js

@@ -21,7 +21,7 @@ export default class UrdfModel {
   joints = {};
   /**
    * @param {Object} options
-   * @param {Element} [options.xml] - The XML element to parse.
+   * @param {Element | null} [options.xml] - The XML element to parse.
    * @param {string} [options.string] - The XML element to parse as a string.
    */
   constructor(options) {