Fix NPE when nested field type has no properties in doc-level monitor

[manaswini1920]

Description

A nested field with type nested but no sub-properties (e.g. "http_request_headers": {"type": "nested"}) causes a NullPointerException during doc-level monitor creation.

In DocLevelMonitorQueries.traverseMappingsAndUpdate(), the code assumes all non-leaf nodes have a properties map. When a field has type: "nested", it's treated as an internal node and the code unconditionally
casts nodeProps["properties"] to MutableMap<String, Any>. If the nested field has no sub-properties, this is null, causing the crash.

Fix: Added a null guard before recursing into nested field properties. Fields with type: "nested" and no properties are now safely skipped during traversal.

Testing: Added unit tests to AlertingUtilsTests covering both cases:

• Nested field without properties — skipped, no exception
• Nested field with properties — sub-fields traversed correctly

Related Issues

Resolves opensearch-project/security-analytics#1472

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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](https://github.com/opensearch-project/alerting/blob/main/CONTRIBUTING.md#developer-certificate-of
-origin).

[opensearch-trigger-bot]

The backport to 2.18 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/alerting/backport-2.18 2.18
# Navigate to the new working tree
pushd ../.worktrees/alerting/backport-2.18
# Create a new branch
git switch --create backport-2049-to-2.18
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9b31fabfced6ece17d80b0891b2cec98d9e9e163
# Push it to GitHub
git push --set-upstream origin backport-2049-to-2.18
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/alerting/backport-2.18

Then, create a pull request where the base branch is 2.18 and the compare/head branch is backport-2049-to-2.18.

[opensearch-trigger-bot]

The backport to 2.19 failed:

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

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/alerting/backport-2.19 2.19
# Navigate to the new working tree
pushd ../.worktrees/alerting/backport-2.19
# Create a new branch
git switch --create backport-2049-to-2.19
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9b31fabfced6ece17d80b0891b2cec98d9e9e163
# Push it to GitHub
git push --set-upstream origin backport-2049-to-2.19
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/alerting/backport-2.19

Then, create a pull request where the base branch is 2.19 and the compare/head branch is backport-2049-to-2.19.