schema validation: disallow static empty values

[steebchen]

Due to prisma/prisma-client-js#318, we should disallow static empty values, such as:

datasource db {
	provider = "sqlite"
	url      = ""
}

This is usually a mistake by the user. It should throw an error that an empty string is not allowed, because otherwise the user will run into a different, most likely unclear error.

Maybe we could even apply this behavior for all empty static strings, not just datasource.url.