fix: Cannot read property 'tagName' of null

Have a look at this issue #1154 .
https://codesandbox.io/s/loving-matsumoto-hp985?file=/index.html
Open devtools and click the *blank space* of svg(not path) will reproduce this issue.

`parentNode` may be `null`.
https://github.com/docsifyjs/docsify/blob/develop/src/core/router/history/html5.js#L25-L27

However, hash router has fixed this issue😜.
https://github.com/docsifyjs/docsify/blob/develop/src/core/router/history/hash.js#L47-L49

src/core/router/history/html5.js

@@ -24,7 +24,7 @@ export class HTML5History extends History {
     on('click', e => {
       const el = e.target.tagName === 'A' ? e.target : e.target.parentNode;
 
-      if (el.tagName === 'A' && !/_blank/.test(el.target)) {
+      if (el && el.tagName === 'A' && !/_blank/.test(el.target)) {
         e.preventDefault();
         const url = el.href;
         // solve history.pushState cross-origin issue