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

https://jira.spring.io/browse/AMQP-447

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Hi @panchenko !

First of all thank you for the contribution!
We are always glad to see any feedback from the community and realy are happy to see PRs :smile: 

To be honest I don't see difference with previous state.
Before this change we had a fallback to the `InetAddress`:
````java
if (host == null || host.length() == 0) {
     InetAddress[] ret = new InetAddress[1];
     ret[0] = impl.loopbackAddress();
     return ret;
}
````
And I guees it is `localhost`.

Right now you just ignore an empty string, so we end up with:
````java
else {
				return new SimpleConnection(this.rabbitConnectionFactory.newConnection(this.executorService),
									this.closeTimeout);
			}
````
Which uses `com.rabbitmq.client.ConnectionFactory#DEFAULT_HOST = "localhost"` as well.

So, which goal do you pursue with this change?