Fixed bug from calling $logout() multiple times

@katowulf - This fixes #431. We used to set `this._currentUserData` to `undefined` every time `$logout()` was called. This worked in most cases since the called to `this._authClient.logout()` ended up firing out `_onLoginEvent()` callback which set `this._currentUserData` to `null` and resolved any leftover `getCurrentUser()` promises. However, if we called `this._authClient.logout()` when we were already logged out, Simple Login does not fire the `_onLoginEvent()` callback and `this._currentUserData` gets stuck as `undefined`, which causes problems for logging in and getting the current user. Having `this._currentUserData` as `undefined` should only be valid at the very initializing of this service. It should always otherwise be `null` or a valid user object. As a result, this change just sets it to `null` when we call `$logout()`.

cc/ @davideast