Do not send custom confirmation instructions unless user is persisted

What does this MR do and why?

An error was raised when Identity Verification is enabled and a user can not be persisted during sign up due to validation errors.

Closes https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/218

How to set up and validate locally

1. Enable identity verification

   Feature.enable(:identity_verification)

2. Create a new account with a weak password (such as password) on http://localhost:3000/users/sign_up

3. Verify no error is raised

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.