Fix create method of registrations_controller

What does this MR do and why?

Failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/7529384

rspec ./jh/spec/features/users/signup_with_phone_spec.rb:102 # Signup with phone with SaaS environment with invalid password renders error message

Upstream removed yield new_user if block_given?. This causes JH's block not to be called.

Changes from: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/119368

Screenshots or screen recordings

After fixing:

How to set up and validate locally

1. Simulate SaaS
2. Turn on FF registrations_with_phone
3. Turn on ApplicationSetting phone_verification_code_enabled
4. Goto http://localhost:3000/users/sign_up page
5. Fill in a weak password
6. Fill in a correct verification code
7. Click register button

MR acceptance checklist

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