Fixing for NPE when there is no query specified for the standard retriever

[pmpailis]

query parameter for a standard retriever is optional as per the documentation.

However, it is possible to run to an NPE when using an "empty" standard retriever within a compound retriever like rrf or linear, as there is no guard against null values when computing topDocsQuery (in contrast to what we do in other methods).

In this PR, we assume that an empty/null body equals to a MatchAllDocsQuery (as in standard search API) and capture that only for topDocsQuery. There were a couple of other options visited, like handling this on rewrite, or returning null, but the scope of the fix was changing significantly (e.g. we would also have to update all consumers of topDocsQuery including RuleQueryBuilder) so this seemed like the most constrained, yet sufficient, solution. Happy to discuss more on this though.

Closes #142336

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @pmpailis, I've created a changelog YAML for you.

[iverase]

oh, it was just that, nice. LGTM

[ioanatia]

changes make sense to me.
consider that this is already valid and executes a match all:

GET wikipedia/_search
{
    "retriever": {
        "standard": {
        }
    }
}

I don't see why it shouldn't be supported when using empty standard retrievers in compound retrievers.

[ioanatia]

do we need a NodeFeature here for bwc tests? I think only for these tests, since for the one in 10_standard_retriever.yml that should already work on main and previous versions (we just did not have a test for it yet).

[pmpailis]

Good point! Added in 5294f8b

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
✅	9.3
❌	8.19	Commit could not be cherrypicked due to conflicts
✅	9.2

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 142479

[pmpailis]

💚 All backports created successfully

Status	Branch	Result
✅	8.19

Questions ?

Please refer to the Backport tool documentation