MINOR: connection: Be prepared to handle conn-stream with no connection

MINOR: connection: Be prepared to handle conn-stream with no connection

The conn-stream will progressively replace the stream-interface. Thus, a
stream will have to allocate the backend conn-stream during its
creation. This means it will be possible to have a conn-stream with no
connection. To prepare this change, we test the conn-stream's connection
when we retrieve it.