resolve NRE with AmpqConnection

[Aaronontheweb]

Changes

close #1659 - a simple null check is all that was required here.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [x] This change follows the Akka.NET API Compatibility Guidelines.
• [x] This change follows the Akka.NET Wire Compatibility Guidelines.
• [x] I have reviewed my own pull request.
• [x] Design discussion issue #1659

[Aaronontheweb]

I would like to have some real test coverage here - working on that via https://github.com/akkadotnet/Alpakka/pull/1890