sampler returns nulls in CSV

[SEKIRO-J]

Fixes #8845.

Description

When parsing a CSV in the sampler, if the whole column values are not provided but column header is provided, this column will be ignored.

Ideally we still want to return this column with values of null or "".
In this implementation it will return ""

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths.
• added integration tests.
• been tested in a test Druid cluster.

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[jon-wei]

I think this should just return nulls as is, any null->empty string conversion should have happened already at this point, so any nulls you see here should really be null

[gianm]

I restarted the failed integration test. I'm not sure if it's transient or not, but we'll see.