Default settings to None where appropriate

[jbkolze]

Should default settings to None in most cases.

Handle missing settings / env vars at boundary so that they are identified early and don't appear at runtime.

Use @property annotations when needed in settings, e.g. fastapi_root_path

[jbkolze]

Referring specifically to this: https://github.com/USACE-WaterManagement/cwms-batch-events/blob/cwbi-dev/cwms_batch_events/core/settings.py