[Bug]: `dab validate` should at least `warn` when zero entities are in the config.

[JerryNixon]

What?

dab validate returns "Config is valid". when there are zero entities.

• entities can be added by autoentities
• entities can be added by entities

But when autoentities is not configured and entities is empty, validate should warn:

[WARN] No entities are defined in this configuration.

Important

This is NOT an error because it could be the top-level configuration in a multi-config environment.