Don't allow nil value for allow_force_push

What does this MR do and why?

Contributes to #392871 (closed)

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/25340

Problem

Protected Branch API allows to set null for allow_force_push parameter. It raises a database exception.

Solution

Use allow_blank: false to prevent allow_force_push to accept null.

How to set up and validate locally

curl --request PATCH --header "PRIVATE-TOKEN: <your_access_token>" "http://127.0.0.1:3000/api/v4/projects/2/protected_branches/feature-branch?allow_force_push="

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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