[dbapi] fix connect backwards compat harder
Passing None as group and host should result in
"pyro:///cubicweb.appid", not "pyro://None/None.appid". Followup for
2091d275fe5c. Closes #2882666.
[zmq] make publish address optional
It should be possible to use the zmq communication bus in a read-only
manner. Allow setting zmq-address-sub without zmq-address-pub.
Closes #2897178.