refs https://github.com/TryGhost/Team/issues/908
The `cookies` module will unset a cookie if `null` or `undefined` is
passed as the value, or if the value is not passed. The previous call
was passing the options, which were being read as the value, and
resulting in `'[Object object]'` being stored as a cookie.
Explicitly passing `null` as the value makes this code correct and
easier to maintain.
