Improve Threat Monitoring backend integration

What does this MR do?

This improves the integration with the backend for WAF statistics reporting in a few ways.

Part of WAF statistics reporting.

In particular, this:

• Handles 404 responses from the WAF statistics endpoint not as errors, but as an indication that there is no data to display.
• Displays a new empty state when there is no data to display.
• Handles 204 No Content responses from the WAF statisics endpoint, and continues to poll until a 200 OK response (or any error) is received. To do this, the existing pollUntilComplete utility was moved to a more shared location, and tests were added for it.

Screenshots

As the backend isn't ready yet, screenshots can only be reproduced by applying a patch.

The only visual change is the addition of a new empty state in the case that there is no data to display:

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