[Github] "Edit on HackMD" button can't display while the document is empty.


And the following is the error message show in console when I open this page.
```
content.js:80 TypeError: Cannot read property '1' of null
at t (content.js:80)
at content.js:80
at e.trigger (content.js:80)
at e. (content.js:80)
at content.js:80
at Object.next (content.js:80)
at content.js:80
at new Promise ()
at r (content.js:80)
at e.triggerAsync (content.js:80)
```

Start by reproducing the empty-document case for the "Edit on HackMD" button and inspect the console error reported at content.js:80. Done means the button displays when the document is empty and the reported TypeError no longer occurs.

Scritto dal modello di indicizzazione a partire dal testo della issue.