Fix WAGED Instance Capacity NPE during rebalance failures

[GrantPSpencer]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  WAGED Instance Capacity NPE prevents topstate handoff during rebalance failures #3009

Description

• Here are some details about my PR, including screenshots of any UI changes:
  TopState handoff can still occur in response to a node going down even if WAGED rebalance failures are occuring (assuming that there was a previously calculated best possible). However, if a participant is removed from the cluster during rebalance failures, then an NPE will occur which will prevent topState handoff from occurring. This PR adds a test and also proposes a potential fix by protecting instanceCapacity checks from null values.

Tests

• The following tests are written for this issue:[
  Added test testNPEonRebalanceFailure to class TestWagedNPE.java and did minor refactoring of the test class.

• The following is the result of the "mvn test" command on the appropriate module:
  [INFO] ------------------------------------------------------------------------
  [INFO] BUILD SUCCESS
  [INFO] ------------------------------------------------------------------------
  [INFO] Total time: 31.493 s
  [INFO] Finished at: 2025-03-13T17:02:12-07:00
  [INFO] ------------------------------------------------------------------------

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)

[junkaixue]

The interesting thing is that shall we skip this instance and continue or fail the entire pipeline using prev compute.