Allow null to be passed into RabbitProperties.setAddresses

[quaff]

Before this commit, RabbitProperties cannot be copied when addresses is null.

[wilkinsona]

Thanks for the PR, @quaff, but I'm not sure that we should make this change for a couple of reasons:

1. @ConfigurationProperties classes are not considered to be public API and the binder will never call the setter with null
2. I suspect there may be other @ConfigurationProperties classes that assume that values passed into setters will not be null

We need to consider 1 and decide if it's something that we want to support. We'd then need to do a review of the codebase to apply any change consistently. I think that's better done as part of #10712. In the meantime, calling code that's chasing to interact with RabbitProperties directly can perform its own null check when mapping the result of getAddresses() on one instance to setAddresses() on another. You could use Boot's PropertyMapper and alwaysApplyingWhenNonNull() to help with that.