ESQL: Should MV_APPEND return null if any value is null?

[alex-spies]

# curl -u elastic:password -H "Content-Type: application/json" "127.0.0.1:9200/_query?format=txt" -d '
{
  "query": "row x = [1,2], y = null | eval z = mv_append(x, y)"
}'

       x       |       y       |       z       
---------------+---------------+---------------
[1, 2]         |null           |null

This behavior is undocumented and I wonder if it's even useful. As a user, I expected MV_APPEND to return the left hand side multi-value rather than null, i.e. z = [1,2].

This is particularly tricky because we'd like to enhance MV_APPEND's behavior to take an arbitrary amount of arguments in #114436.
If I wanted to append multiple multivalues into a single one, simply skipping null values, there won't be a good way to achieve this. For just 2 arguments, one can work around this via CASE, but even that is not very ergonomic.

We should either make MV_APPEND an exception where one null value doesn't make us return null overall; or we should clearly document the behavior, including examples from our csv tests.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticsearchmachine]

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

[nik9000]

I'd be ok making MV_APPEND treat null like an empty list of values. It is that at a lot of places in Elaticsearch.