Fix default value handling in REGEXP_EXTRACT transform function

[yashmayya]

• Fixes https://github.com/apache/pinot/issues/14488.
• REGEXP_EXTRACT(stringCol, 'nonMatchingRegex', 0, 'null') returns nonMatchingRegex instead of null.
• REGEXP_EXTRACT('stringLiteral', 'nonMatchingRegex', 0, 'null') correctly returns null however.
• This is because the corresponding transform function has a bug here - https://github.com/apache/pinot/blob/c096c705903b40fa8f951958a2b14bfd524076e9/pinot-core/src/main/java/org/apache/pinot/core/operator/transform/function/RegexpExtractTransformFunction.java#L83-L88
• The corresponding scalar function is correct though - https://github.com/apache/pinot/blob/c096c705903b40fa8f951958a2b14bfd524076e9/pinot-common/src/main/java/org/apache/pinot/common/function/scalar/StringFunctions.java#L280-L289
• This patch fixes the transform function bug and adds a small unit test.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.76%. Comparing base (59551e4) to head (6173c77). Report is 1355 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #14489 +/- ## ============================================ + Coverage 61.75% 63.76% +2.01% - Complexity 207 1565 +1358 ============================================ Files 2436 2667 +231 Lines 133233 146442 +13209 Branches 20636 22467 +1831 ============================================ + Hits 82274 93380 +11106 - Misses 44911 46144 +1233 - Partials 6048 6918 +870 ``` | [Flag](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | Coverage Δ | | |---|---|---| | [custom-integration1](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `100.00% <ø> (+99.99%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `100.00% <ø> (+99.99%)` | :arrow_up: | | [integration1](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `100.00% <ø> (+99.99%)` | :arrow_up: | | [integration2](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `0.00% <ø> (ø)` | | | [java-11](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.73% <100.00%> (+2.03%)` | :arrow_up: | | [java-21](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.64% <100.00%> (+2.01%)` | :arrow_up: | | [skip-bytebuffers-false](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.76% <100.00%> (+2.01%)` | :arrow_up: | | [skip-bytebuffers-true](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.61% <100.00%> (+35.88%)` | :arrow_up: | | [temurin](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.76% <100.00%> (+2.01%)` | :arrow_up: | | [unittests](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `63.76% <100.00%> (+2.01%)` | :arrow_up: | | [unittests1](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `55.54% <100.00%> (+8.65%)` | :arrow_up: | | [unittests2](https://app.codecov.io/gh/apache/pinot/pull/14489/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache) | `34.07% <0.00%> (+6.34%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=apache#carryforward-flags-in-the-pull-request-comment) to find out more.

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