Fix CSV downloads for multiple series in the same chart

What does this MR do?

This MR fixes cases in which the "Dashboard Panel" failed rendering when data was undefined/missing from the results, such as when data is loading or fails to load.

Additionally is exports a complete dataset of values for each metric in the panel, so it covers cases in which our data has more complex shapes, like when multiple metrics/series are used.

Screenshots

No changes in UI, but given this panel with multiple series:

We get different CSV results:

Before	After (all data columns)

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

Closes #214627 (closed)