Fix for export projection and null data excluded bugs

[inv-jishnu]

Description

This PR is to fix 2 bugs found during testing, The issues were raised and fixed by @thongdk8 (Thank you). I have added the fix to this branch along with more unit tests to check and confirm the fix

• The import data file is empty when projections are not specified.
• For Json and JsonLines format, if a column values is null it excluded from export file.

Related issues and/or PRs

NA

Changes made

The following issues are fixed

• The import data file is empty when projections are not specified (Should include all columns).
  For example, if projection argument was not specified with required columns, the generated import file would be empty.
• For Json and JsonLines format, if a column values is null it excluded from export file (Should include columns with null data).
  For example, if a value of a column that is to be imported is null for a particular row of data, the column is not added for that data in the import data file. Let say table sample has columns a, b, c, consider a to be the partition key. Let say for a=1, the respective value of b is null, hence the entry for a=1, in the import file b will be excluded and only a and c will be added to it.

Checklist

The following is a best-effort checklist. If any items in this checklist are not applicable to this PR or are dependent on other, unmerged PRs, please still mark the checkboxes after you have read and understood each item.

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes.
• I have considered whether similar issues could occur in other products, components, or modules if this PR is for bug fixes.
• Any remaining open issues linked to this PR are documented and up-to-date (Jira, GitHub, etc.).
• Tests (unit, integration, etc.) have been added for the changes.
• My changes generate no new warnings.
• Any dependent changes in other PRs have been merged and published.

Additional notes (optional)

This is a bug fix PR for data loader export part

Release notes

NA

[Copilot]

Pull Request Overview

This PR fixes two export bugs by ensuring that (1) when projections are not specified, all columns are exported and (2) null column values are correctly included in the output across JSON, JSONLines, and CSV formats.

• Added tests for partial projections with and without metadata.
• Updated producer tasks to correctly handle cases when no projections are provided and when a column value is null.
• Enhanced UnitTestUtils to support partial output data for more granular testing.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
data-loader/core/src/test/java/com/scalar/db/dataloader/core/dataexport/producer/JsonProducerTaskTest.java	Added tests covering both projection scenarios and metadata inclusion/exclusion for JSON export.
data-loader/core/src/test/java/com/scalar/db/dataloader/core/dataexport/producer/JsonLineProducerTaskTest.java	Added tests for JSONLines export with various projection and metadata combinations.
data-loader/core/src/test/java/com/scalar/db/dataloader/core/dataexport/producer/CsvProducerTaskTest.java	Added tests for CSV export with similar projection and metadata scenarios, though test method names reference JSONLine incorrectly.
data-loader/core/src/test/java/com/scalar/db/dataloader/core/UnitTestUtils.java	Introduced helper methods to generate partial output data and projected column lists.
data-loader/core/src/main/java/com/scalar/db/dataloader/core/dataexport/producer/JsonProducerTask.java	Updated column projection check and added null-handling logic using NullNode.
data-loader/core/src/main/java/com/scalar/db/dataloader/core/dataexport/producer/JsonLineProducerTask.java	Made similar projection and null-handling updates as in JsonProducerTask.
data-loader/core/src/main/java/com/scalar/db/dataloader/core/dataexport/producer/CsvProducerTask.java	Updated the projection check to ensure all columns are exported when no projection is specified.

Comments suppressed due to low confidence (1)

data-loader/core/src/test/java/com/scalar/db/dataloader/core/dataexport/producer/CsvProducerTaskTest.java:64

• The test method name references 'JsonLineString' even though it belongs to a CSV producer test. Consider renaming it to '...ShouldReturnValidCsvString' for clarity.

@Test void process_withValidResultList_withPartialProjections_shouldReturnValidJsonLineString() {

[brfrn169]

LGTM! Thank you!

[ypeckstadt]

LGTM. Thank you.

[komamitsu]

LGTM! 👍

[feeblefakie]

LGTM! Thank you!