AMQP-447 Ignore empty string passsed to AbstractConnectionFactory.setAddresses()

https://jira.spring.io/browse/AMQP-447

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Why really don't throw `IllegalArgumentException` as @michaelklishin suggests?
I understand that the main fix should be provided in the RabbitMQ client library, but it won't hurt to have double check in the Spring AMQP for previous Client versions.

From other side it might be a breaking change for point release, where someone can rely on that `empty string` behaviour and do falback to the localhost. E.g. through the the `property-placeholder`, when he use `amqp.addresses=` property for the development environment.
WDYT?