another mathjax 4 fix (#86)

* version

* fix null element

Author: jivey

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openstax/highlighter",
-  "version": "1.16.0",
+  "version": "1.16.1",
   "main": "dist/index.js",
   "license": "MIT",
   "files": [

src/dom.ts

@@ -226,6 +226,7 @@ export default function dom(el: any) {
     },
 
     matches(selector: any) {
+      if (!el) { return false; }
       const method = el.matches || el.mozMatchesSelector || el.msMatchesSelector || el.oMatchesSelector || el.webkitMatchesSelector;
       return (method != null ? method.call(el, selector) : undefined);
     },