return undefined instead of an emtpy object

[51smith]

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Hi Aaron

As requested, a PR on the development branch.

Kind regards,
Shane

Meteor login handler runner expects either and object with an user id, or an error, otherwise you are not supposed to return anything, or undefined, so it can continue with other login handlers.
Currently we are returning an empty object, when we are sure the login attempt is not meant for this handler. The empty object however will actually cause an exception and stop the handler from running more login attempts later in the chain and throw an error instead.

Account_server.js:318

throw new Error("A login method must specify a userId or an error");