Creating new schema with `undefined` rule should fail

[iamvanja]

Consider this scenario:

const schema = new Schema({ email: undefined })

This should IMHO fail or at least spit out a warning.

Now validating values against this schema returns an empyy object as if validation is passed.

// output: {}
schema.validate({ email: 'aaaa' })

If creating new schema did not fail, this should certainly fail.

I don't mind working on this if there is interest and agreement how this should be handled.