Allow empty directives for the CSP

What does this MR do?

Related issues

No issues but this is blocking gitlab-com/gl-infra/k8s-workloads/gitlab-com!1613 (merged) and progress on gitlab-org&6363

There is a check that makes sure directives are set when the content security policy (CSP) is enabled, however the product now supports that config and generates a secure CSP automatically. We're deploying this configuration to staging and would eventually like this to be enabled by default for self-managed and even .com.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated (Inspired by gitlab-org/omnibus-gitlab!5920 (merged))
• Tests added
• Integration tests added to GitLab QA
• [-] Equivalent MR/issue for omnibus-gitlab opened Omnibus already supports this config