ESQL: Fix AttributeSet#add() returning the opposite expected value

[ivancea]

Set/Collection#add() is supposed to return true if the collection changed (If it actually added something). In this case, it must return if the old value is null.

Extracted from https://github.com/elastic/elasticsearch/pull/114317 (Where it's being used)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[ivancea]

@alex-spies In CombineProjections we aren't using the return value (Until the Categorize PR), so all nice! Thanks for checking! If tests are ok (And in the other PR, they were), we should be fine then :blob:

[nik9000]

Thanks for yanking it into it's own PR!

[ivancea]

Also updated the QL counterpart. I didn't find any usage of the add(), at all

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x