INT-3549 Fix AMQP o-c-a Validation

JIRA: https://jira.spring.io/browse/INT-3549

`afterPropertiesSet()` checks for `NullChannel` to determine
whether a correlation expression is needed for confirms.

This code fails when `nullChannel` is proxied.

Extract the type before testing.
Need to search out through the whole project for similar issues...
Not related to this PR, but we just should bear in mind