Can't join a room as a guest if email is a required 3pid for registration

This issue has been migrated from [#4146](https://github.com/matrix-org/synapse/issues/4146).

---

### Description

I cannot join a room as a guest if email is a required 3pid for registration.

### Steps to reproduce

- Ensure the following is in `homeserver.yaml`
```
enable_registration: true
registrations_require_3pid:
- email
allow_guest_access: true
enable_registration_captcha: true
recaptcha_public_key : "REDACTED"
recaptcha_private_key: "REDACTED"
```
- attempt to join room as guest

I expect to be able to join the room as a guest, but I get `No appropriate authentication flow found`.
If I remove `email` from `registrations_require_3pid` I am able to join as a guest successfully.

### Version information

- **Homeserver**: Self-hosted

If not matrix.org:
- **Version**: v0.33.8
- **Install method**: docker
- **Platform**: Running in Kubernetes behind ingress controller (reverse proxy)