Fix: Removing all domains does not reset primary domain

What does this MR do and why?

Fix: Removing all domains does not reset primary domain

Related: #514767 (closed)

Changelog: fixed

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Before	After

How to set up and validate locally

1. Initialize a generic Pages/Plain HTML project from template
2. Run a pipeline to deploy pages
3. Go to Deploy > Pages > New Domain
4. Enter in any fake domain and ignore the SSL requirements. Save Changes.
5. Select the new fake domain under the Primary domain dropdown. Save Changes.
6. Delete the newly created fake domain under Domains
7. Use the API endpoint https://gitlab.com/api/v4/projects/PROJECT_ID/pages to check the pages_primary_domain. It should be reset.