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

https://jira.spring.io/browse/AMQP-447

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I'd say that these tests don't reflect the requirement of the change. 
We really should demostrate here the falback to `host/port` when we provide empty `addresses`.
I think we can `mock` `com.rabbitmq.client.ConnectionFactory#newConnection(ExecutorService executor, Address[] addrs)` to varify `addrs`  argument.