Allow blank external_dashboard_url

What does this MR do?

Note: This is currently pointed at rc/add_dashboard_timezone. Once that is merged this will be pointed at master

Part of #214370 (closed), also addresses #220210 (closed)

!33120 (merged) removed the database not null constraint. This MR removes the application not null constraint. This is needed since we added a new field dashboard_timezone and users will have cases where they update dashboard_timezone while still leaving external_dashboard_url blank.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team