Insights: Only display page config with valid values

What does this MR do?

When writing charts and dashboards in the native insights yml file. It's useful to be able to declare reusable fields via YAML anchors.

When an invalid page declaration is present in the yml file. An empty item is displayed in the dropdown list

This Merge Request aims to filter out pages in the config that don't declare both a title and a charts field.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry N/A Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides N/A Database guides
• 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

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