Fix outbound_local_requests_whitelist nil for existing ApplicationSetting records

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/65096 by adding a column default

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
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• [-] Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers