Ensure that pages metadata exists when updating it

What does this MR do?

This fix is a bit paranoid: pages_metadata was added in 12.* and created via background migration.

Now it should exists for every project. (this is true for .com)

But if users just upgrade to 13.* after a long wait, then background migration may still be in-progress, so we want to ensure that metadata exist before we update it with deployment_id

This thought popped up while I was implementing #254222 (closed)

Screenshots (strongly suggested)

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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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