Show alerts when prometheus service is active

What does this MR do?

The alert list is sometimes not shown when the prometheus alerts setting is enabled. This MR fixes that issue, and updates the help message shown when the list is empty.

It is paired with these docs changes: !34241 (merged)

Discussion about the proposed solution: #218639 (comment 355823730)

Issue: #218639 (closed)

Screenshots

	Before	After
With prometheus alerts on + alert integration off
With an empty list

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required) here: !34241 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] 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
• [n/a] 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:

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team