Creating new schema with `undefined` rule should fail

Consider this scenario:
```js
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.
```js
// 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.

Start by reproducing the behavior at the Schema constructor and validate entry points shown in the issue. Clarify whether an undefined rule should be rejected during schema creation or validation, then add coverage for the agreed behavior and confirm that validating the example no longer silently returns an empty object.

Written by the indexing model from the issue text.