Use old `id_token` if new `id_token` not in refresh response

[wryrye]

Description

In the case where a new id_token is not present in a refresh response, let's avoid setting id_token to undefined and instead use the old id_token!

References

Issue: #628

Testing

1. Make a refresh request (req.oidc.accessToken.refresh()) where the response is missing an id_token
2. Confirm subsequent accesses of req.oidc.idToken and req.oidc.user are NOT undefined

• [ ] This change adds test coverage for new/changed/fixed functionality

Checklist

• [ ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [ ] All active GitHub checks for tests, formatting, and security are passing
• [ ] The correct base branch is being used, if not the default branch