It's niche, but if no replication base url was configured and a peer was
configured with a NULL port in the db, then we would try to format None
as an integer and get a TypeError for our troubles.

Spotted by work in progress elsewhere to make Sydent pass mypy --strict.

Fixes #419.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

I wonder if mypy would have spotted this---surprised flake8 didn't!