Fix NotNullViolation in issues API

What does this MR do and why?

Contributes to #393030 (closed)

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1915

Problem

It's possible to provide an empty value for "confidential" field. It raises an exception because of constraint violation.

Solution

Use allow_blank to prevent the submission of empty values for confidential field.

How to set up and validate locally

curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "http://127.0.0.1:3000/api/v4/projects/2/issues?title=issue&confidential="

Before

{"message":"500 Internal Server Error"}

After

{"error":"confidential is empty"}

MR acceptance checklist

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