null is an object bugfixes

A relation can sometimes be set to null. In JS, `typeof null === 'object'`, and
a condition wasn't taking that case into account, allowing `Object.keys` to be
called on `null` (non object).

Continues work over 300cae4c7845c6e29d35d33f7a6c811451541f35.
See: https://github.com/orbitjs/orbit.js/pull/41