Improve config.ts validation

[emilgoldsmith]

Expected Behavior

It should validate everything strictly and correctly

Current Behavior

I for example had not provided a ROOT_DIRECTORY variable in CI environment and that should've errored, but it didn't because it just checks for undefined and not for empty string right now

Possible Solution

I would change the validateString function to also somehow take environment into account and enforce that nothing is empty string except for in development where some things like AWS credentials can be an empty string (but things like DB password and ROOT_DIRECTORY still shouldn't)