fix: `additionalProps` should be conditional on the `UserAuthCard`

[francesconi]

This PR addresses two minor issues:

1. Previously, after creating a login flow, its ID was not included in the query parameters, unlike the other flows.
2. The signup URL was always rendered, even when it was undefined. This resulted in an unclickable link. Additionally, this change allows users who haven't enabled registration to hide the URL.

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [ ] I have referenced an issue containing the design document if my change introduces a new feature.
• [x] I have read the security policy.
• [x] I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security vulnerability, I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added the necessary documentation within the code base (if appropriate).

[CLAassistant]

All committers have signed the CLA.