fix: regexp_extract returns match in mismatched group

[HolyLow]

The implementation of Re2Extract has a bug, that it might consider a mismatched group as MATCHED empty string "" rather than MISMATCHED std::nullopt.

For example, in the function calling: regexp_extract("rat cat\nbat dog", "ra(.)|blah(.)(.)", 2).
In this case, for group 2 the result must be std::nullopt because no substring would match pattern blah(.).
But the current implementation would mistake the matching of group 1 ra(.) as a empty match case for group 2, and thus return a empty matching, which is wrong.

This PR fix this bug in Re2Extract implementation.

Also note that this bug behavior exists in Re2ExtractAll as well, but this PR doesn't modify in Re2ExtractAll because existing UTs of Re2ExtractAll already rely on this behavior.

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[HolyLow]

@kgpai @mbasmanova Could you kindly help review this PR? Thanks a lot.

Any suggestion is welcome.

[mbasmanova]

Also note that this bug behavior exists in Re2ExtractAll as well, but this PR doesn't modify in Re2ExtractAll because existing UTs of Re2ExtractAll already rely on this behavior.

Sounds like both the implementation is buggy and test expectations are wrong. In this case we need to fix both the implementation and the test.

Would you check Presto Java to see if Velox behavior matches it?

[mbasmanova]

Looks like Presto Java returns NULL:

presto:di> SELECT regexp_extract('rat cat\nbat dog', 'ra(.)|blah(.)(.)', 2);
 _col0
-------
 NULL
(1 row)

@HolyLow Would you create a GitHub issue to describe this problem? Then, reference the issue in the PR description.

[mbasmanova]

Thank you for the fix!

CC: @amitkdutta @kevinwilfong @kagamiori

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