[BUG] Omitting the `database` param in a target fails silently.

[zprobst]

Describe the bug
If you describe a target and accidentally leave off the database param, you will get a NullDatabaseConnector for a target. This is meant mostly for internal or debugging use, which when accessing a target from a project, should likely warn the user. Not convinced it is technically wrong to create a null target. Its a good way to make sure that the rest of your pipeline is running okay and free of error, but it should only be accessed through explicit configuration.