The current implementation needs an initial parameter set that points to a valid map during on_reconfigure. If the parameters are not set, on_configure fails and the load_map_service is not started:

https://github.com/ros-planning/navigation2/blob/3e4dd6fa24f4d155d962b1dc9ba92110ef121641/nav2_map_server/src/map_server/map_server.cpp#L97

This imho limits the usability because we can't start the map server until we know which map we want to use.

I could write a short PR that starts the services before checking the parameters and remove the thrown exception if there was no valid parameter set.