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

https://jira.spring.io/browse/AMQP-447

I have signed and agree to the terms of the SpringSource Individual Contributor License Agreement
Overall, this is about undertanding why there are 2 ways to set host(s) and documenting how they should work together.
Another possible backwards compatible solution is
```java
if (!hasText(addresses)) {
  setHost(DEFAULT_HOST);
  setPort(DEFAULT_PORT);
  this.addresses = null;
  return;
}
```
and adding the corresponding documentation.