ESQL: FoldNull folding aggs into literals, raising an "unknown agg" error about elasticsearch HOT 6 OPEN

Related with #100634
I'm closing as duplicate. It's not exactly the same issue I think, but it's part of it for sure

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median(const) is already implemented, while percentile still needs a corresponding MV_... function to act as surrogate.

Not sure it's worth adding a validation for this, as implementing something like MV_PERCENTILE (but not exposing it to users, internal usage only) may not be a whole lot more work.

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

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I'm not sure we should close this, #100634 is a request for enhancements while this one is an actual bug (error 500).
Waiting for #100634 to be fully supported, we should at least add some validation for these cases.

Interestingly, median(null) works just fine, while percentile(null, 50) throws an error

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I'm 50/50 here really. The aggs that don't fail, is because they have the surrogate, indeed. However, any agg with multiple parameters, that have a null in one of them but a non-foldable in the other, will fail.

For example, percentile can't really be fixed. We can't convert PERCENTILE(field, null) to MV_PERCENTILE, as field is not foldable (At least not with the current methods, afaik). And it fails too.

So, we can reopen this if you think it's interesting

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Ah, I was thinking about percentile(null, 50), but this is indeed a different problem than percentile(some_field, null). The latter indeed should end up with an error thrown at validation time.

Re-opening, but I think we need to make the wording of this issue more precise.

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