Simplified Linear & RRF Retrievers - Return error on empty fields param

[Mikep86]

Updates the simplified linear & rrf retrievers to return an error when the fields param is empty:

GET movies/_search
{
  "retriever": {
    "linear": {
      "fields": [],
      "normalizer": "minmax",
      "query": "Harry Potter"
    }
  }
}

Previously, this would query all fields specified by index.query.default_field, but this creates a potential future problem with supporting combinations of query and retrievers, like this:

GET movies/_search
{
  "retriever": {
    "linear": {
      "fields": [],
      "normalizer": "minmax",
      "query": "Harry Potter",
      "retrievers": [
        {
          "retriever": {
            <your custom sub-retriever here>
          }
        }
      ]
    }
  }
}

In contrast, this should (at most) only execute the custom sub-retrievers and not query any fields for "Harry Potter". Returning an error for now when the fields param is empty gives us more options for how to combine query and retrievers in the future.

[elasticsearchmachine]

Pinging @elastic/search-eng (Team:SearchOrg)

[elasticsearchmachine]

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

[elasticsearchmachine]

Hi @Mikep86, I've created a changelog YAML for you.

[kderusso]

LGTM

[Samiul-TheSoccerFan]

LGTM

[Mikep86]

@elasticmachine update branch

[Mikep86]

@elasticmachine update branch

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.19