Handle undefined `connection_details.peer_{host,port}` in GET /api/channels responses

[michaelklishin]

AMQP 0-9-1 shovels can use a special Erlang client known in RabbitMQ as the direct client.

That client uses Erlang inter-node connectivity and won't have some typical socket stats, namely peer_host and peer_port, exposed under connection_details.

The problem is that an undefined value will be rendered as "undefined" and not a null or a missing field.