[backport 2.x] Rollup Interceptor sourceIndex NPE fix (#773)

[petardz]

Issue #, if available: #773

Description of changes: manual backport

CheckList:

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

Merging #781 (c7d632a) into 2.x (86339f9) will increase coverage by 0.07%.
The diff coverage is 65.00%.

@@             Coverage Diff              @@
##                2.x     #781      +/-   ##
============================================
+ Coverage     75.68%   75.75%   +0.07%     
- Complexity     2643     2655      +12     
============================================
  Files           337      337              
  Lines         15552    15560       +8     
  Branches       2252     2254       +2     
============================================
+ Hits          11771    11788      +17     
+ Misses         2472     2462      -10     
- Partials       1309     1310       +1     

Impacted Files	Coverage Δ
.../org/opensearch/indexmanagement/util/IndexUtils.kt	67.01% <ø> (-9.41%)	⬇️
...management/rollup/interceptor/RollupInterceptor.kt	77.77% <65.00%> (-2.37%)	⬇️

... and 11 files with indirect coverage changes

[opensearch-trigger-bot]

The backport to 2.5 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-2.5 2.5
# Navigate to the new working tree
cd .worktrees/backport-2.5
# Create a new branch
git switch --create backport/backport-781-to-2.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 fa00e92727786301b3d82431f56d34ff93ed7624
# Push it to GitHub
git push --set-upstream origin backport/backport-781-to-2.5
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.5

Then, create a pull request where the base branch is 2.5 and the compare/head branch is backport/backport-781-to-2.5.