[Fix] add non-null check for queryBuilder in NeuralQueryEnricherProcessor

[zhichao-aws]

Description

In #337 and #541 we support the default model id for neural and hybrid query. However, in current implementation, after we set up a search pipeline to support default model id, we can not search with empty query body (i.e. GET /index_name/_search or POST /index_name/_search). We'll get exception like below:

TransportError(500, 'null_pointer_exception', 'Cannot invoke "org.opensearch.index.query.QueryBuilder.visit(org.opensearch.index.query.QueryBuilderVisitor)" because "queryBuilder" is null').

This PR fix this exception by adding a non-null check in NeuralQueryEnricherProcessor.

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed as per the DCO using --signoff

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.44%. Comparing base (ab59d66) to head (38eda5f).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #615   +/-   ##
=========================================
  Coverage     84.43%   84.44%           
- Complexity      603      604    +1     
=========================================
  Files            48       48           
  Lines          1825     1826    +1     
  Branches        275      276    +1     
=========================================
+ Hits           1541     1542    +1     
  Misses          161      161           
  Partials        123      123           

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[zhichao-aws]

Seems there is a failed test about model deployment. I think this is a flaky test not related to code changes here.

For bwc failures, it's not related to code changes here neither. We can find the same failure in #602

[zhichao-aws]

rebase main

[zhichao-aws]

All checks have passed. @vibrantvarun Could you please help review this and #614 ?

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-615-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 b97dbe80f0b1dd8578a2dcdbed702c7660849747
# Push it to GitHub
git push --set-upstream origin backport/backport-615-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-615-to-2.x.