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

https://jira.spring.io/browse/AMQP-447

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Not sure that this implicit `DEFAULT_` is good. We lose the reason of this change.
I think we don't have choice, unless provide this minor breaking change and do fallback to the provided `host/port`, when `addresses` is empty.
For those users who haven't noticed this issue before it will look similar, because we did falback to `localhost` with the current state and it was good for them. And even with this change we will do falback to `host/port`. But if they aren't provided, they are for `localhost` too.