Implement buildEmptyAggregations for MultiTermsAggregator (#7089)

[austintlee]

Description

The implementation for the 'buildEmptyAggregation' method for MultiTermsAggregator was incomplete and was simply returning null. This change completes the implementation for this method and adds a YAML REST test case for it.

I have another PR that has a unit test for this change. I want to get feedback on that separately from this PR.

Issues Resolved

#7089
#5785

Check List

• New functionality includes testing.
  • [x ] All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• [x ] Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14620/
• CommitID: 10531bf
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14622/
• CommitID: ea238f1
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[dblock]

This looks great. Since the unit test detects this exact problem let's roll it in here?

[austintlee]

@dblock I can add the unit test commit to this one. What do you think?

[dblock]

@dblock I can add the unit test commit to this one. What do you think?

Yep, you should.

Iterate this PR to green. I haven't looked at the failures in the gradle check, could be legit. And thank you!

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14715/
• CommitID: 6e9423a
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[austintlee]

Can I get some help with this? I don't know why Gradle Check failed.

[reta]

The Gradle check may fail due to #7470 (#7473 just went into 2.x)

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/15294/
• CommitID: d891f66
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testCancelPrimaryAllocation
      1 org.opensearch.cluster.allocation.AwarenessAllocationIT.testThreeZoneOneReplicaWithForceZoneValueAndLoadAwareness

• URL: https://build.ci.opensearch.org/job/gradle-check/17951/
• CommitID: 1fe4cfd
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testDropPrimaryDuringReplication
      1 org.opensearch.remotestore.RemoteStoreRefreshListenerIT.testRemoteRefreshRetryOnFailure

• URL: https://build.ci.opensearch.org/job/gradle-check/17972/
• CommitID: 0075664
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testNodeDropWithOngoingReplication
      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollCreatedOnReplica
      1 org.opensearch.cluster.allocation.AwarenessAllocationIT.testThreeZoneOneReplicaWithForceZoneValueAndLoadAwareness

• URL: https://build.ci.opensearch.org/job/gradle-check/18627/
• CommitID: 14fd215
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.