fix: remove required check on indices.stats.ShardStats.shards

[LiamMacpherson]

Description

This change addresses the problem wherein we require a field which isn't ever returned inside the "Shards" property on indices stats calls with the level set to "shards".

Issues Resolved

Closes 1174

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[LiamMacpherson]

Left a comment and reminder

[dblock]

Looks good. Please add a test.

[Xtansia]

Thanks for sticking with this @LiamMacpherson, the changes look good, just need to fix the formatting to keep the CI happy.
Could you please run and commit the changes:

./gradlew spotlessApply

[LiamMacpherson]

Ah -- I hadn't noticed that failure! Thanks for highlighting @Xtansia!

Test has been added

[Xtansia]

Thanks @LiamMacpherson

[opensearch-trigger-bot]

The backport to 2.x failed:

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

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
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-1177-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 8a37f3f2776f5ef2244382c6bc379747e05f203e
# Push it to GitHub
git push --set-upstream origin backport/backport-1177-to-2.x
# Go back to the original working tree
popd
# 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-1177-to-2.x.