Add option to swallow empty identifiers

This allows you to specify the `disqualify_empty_identifier: true` option on an experiment, in which case the library won't raise when a subject has an empty identifier, but it will silently disqualify these people instead.

This should fix some issues we are seeing in production for orders without cart_token that are created by the API.

@cjoudrey @CamDavidsonPilon