Check for null to avoid issue #848

[AFAIK](https://developer.mozilla.org/en-US/docs/Web/API/document.querySelector), `document.querySelector` can't return `0` or `false`. This means that [this styleguide entry](https://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml?showone=Tips_and_Tricks#Tips_and_Tricks) applies, and I can simply write `if (documentationLink) { ... }`.