Add empty region when group metrics are missing

What does this MR do?

Display when a panel group is in an empty state in the dashboard

The metrics dashboard didn't let users know when data was missing from a panel. This change is the first step to know why data is missing.

This change makes use of getter to calculate if the a panel group of the dashboard should be displayed as empty.

Pending:

• Use the correct illustration as in the design. Pending: gitlab-svgs!499 (closed)
• Wait for merge of !20818 (merged) to address conflicts here.

Note:

❗ Our issue #34121 (closed) defines multiple empty states to let the use know what went wrong. This is a first pass that should be followed up with more detailed implementation.

Screenshots

In devices

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Loading flow

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

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

Related to #34121 (closed)