Date/Time based Span aggregation should always not present null bucket

[LantaoJin]

Description

| stats agg_func() by span(time, interval)

time-based span aggregation should always not present null bucket:

• composite aggregation with DateHistogramValuesSourceBuilder.missingBucket(false)

Example Data

Name	DEPTNO	birthday
Alice	1	2024-04-21
Bob	2	2025-08-21
Jeff	null	2025-04-22
Adam	2	null

Example Query 1

| stats count() by span(birthday, 1y) as year

Return

count()	year
1	2024-01-01
2	2025-01-01

Example Query 2

| stats count() by span(birthday, 1y) as year, DEPTNO

Return

count()	year	DEPTNO
1	2024-01-01	1
1	2025-01-01	2
1	2025-01-01	null

Example Query 3

| stats bucket_nullable=false count() by span(birthday, 1y) as year, DEPTNO

Return

count()	year	DEPTNO
1	2024-01-01	1
1	2025-01-01	2

Related Issues

Resolves #4318

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[opensearch-trigger-bot]

The backport to 2.19-dev 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/sql/backport-2.19-dev 2.19-dev
# Navigate to the new working tree
pushd ../.worktrees/sql/backport-2.19-dev
# Create a new branch
git switch --create backport/backport-4327-to-2.19-dev
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 2ab5002bac5f9b1ac680412488eeb7e21468147b
# Push it to GitHub
git push --set-upstream origin backport/backport-4327-to-2.19-dev
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/sql/backport-2.19-dev

Then, create a pull request where the base branch is 2.19-dev and the compare/head branch is backport/backport-4327-to-2.19-dev.