SQL: Concat should be always not nullable

[astefan]

This PR fixes the CONCAT function case where, if one of the values is null, the function returns null. Instead it should always consider null as an empty string. Fixes #36169.

[elasticmachine]

Pinging @elastic/es-search

[matriv]

I'd prefer to have an optimizer test for that, actually QueryFolder test to check that the concat is not eliminated because of the FoldNull() rule.

We can keep the integ test too, of course.

[costin]

+1 to what Marios said. The advantage of that unit tests is that it checks the behavior in isolation and thus we can identify in the future if there are any side effects.

[astefan]

run the gradle build tests 1

[matriv]

@astefan The test you added is also nice, but would be great to also add one similar to those: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/optimizer/OptimizerTests.java#L407 that checks that FoldNull() doesn't fold the concat() function to null when at least one null literal is involved.

Sorry for not being more specific before and I confused you with mentioning the QueryFolderTests.

Thank you!

[matriv]

LGTM. Thx for the tests!

[matriv]

Is it already available in TestUtils?