Fix the NPE in stringLast vector aggregation

[abhishekagarwal87]

If a field does not exist, StringLastVectorAggregator will throw NPE.

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  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
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• a release note entry in the PR description.
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• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.