Fix waged instance capacity npe on new resource

[GrantPSpencer]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  NPE occurs for first WAGED resource in cluster after previous WAGED resources have been removed
  WagedInstanceCapacity Null Pointer Exception due to stale _instanceCapacityMap #2891

Description

• Here are some details about my PR, including screenshots of any UI changes:

Full description of the NPE can be found in the issue #2891. Here is a brief summary:

1. _instanceCapacityMap is calculated with WAGED resources in cluster
2. If you remove all WAGED resources, the previous _instanceCapacityMap is still present. This is only recalculated under certain conditions
3. If you add a new WAGED resource, the _instanceCapacityMap is not recalculated before it is used. This stale _instanceCapacityMap can lead to an NPE

This PR addresses the above issue by ensuring the _instanceCapacityMap is null whenever there are no WAGED resources in the cluster. This leads to the map being recalculated before it is used when a new WAGED resource is added.

Tests

• The following tests are written for this issue:
  New test class: TestWagedNPE

• The following is the result of the "mvn test" command on the appropriate module:

$ mvn test -o -Dtest=TestWagedNPE -pl=helix-core

[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 11.836 s - in org.apache.helix.integration.TestWagedNPE
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  35.815 s
[INFO] Finished at: 2024-11-25T10:09:46-08:00
[INFO] ------------------------------------------------------------------------

(If CI test fails due to known issue, please specify the issue and test PR locally. Then copy & paste the result of "mvn test" to here.)

Changes that Break Backward Compatibility (Optional)

• My PR contains changes that break backward compatibility or previous assumptions for certain methods or API. They include:
  N/A

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[GrantPSpencer]

Pull request approved by: @xyuanlu
Commit message: Fix waged instance capacity npe on new resource by clearing the WAGED capacity map whenever there are no WAGED resources in the cluster. This will prevent a stale map from being used once a new resource is added.