Forbid an empty string in key attributes

[ob-ivan]

This is to imitate the DynamoDB behaviour that throws this error:

The AttributeValue for a key attribute cannot contain an empty string value.

I'm using jest-dynalite to test a user creating function, and I'm sad that the unit tests don't fail when creating a user with an empty email despite the email being an index field. I have only found that out in production.

[reconbot]

@tbeseda this seems like a good 4.0 change