Allow to inject falsy values like 'false' and 'null'

Only `undefined` should signify that there is nothing to inject (i.e. nothing was registered) and hence to throw an error; injecting `false` and `null` should be working though.

One of the use cases is to be able to inject `currentUser` into controllers and routes. If the user isn't signed in, it'd be `null`. Currently it throws an error but I actually expected it to work properly (i.e. set `currentUser` on controllers and routes to `null` without throwing anything).