Fix NullPointerException when doing knn search on empty index without dims

[kderusso]

Resolves #111733

Script to reproduce (slightly expanded from bug report issue):

DELETE esre-team-test-chunks

PUT esre-team-test-chunks
{
  "mappings": {
    "dynamic_templates": [],
    "properties": {
      "metadata": {
        "properties": {
          "esre": {
            "properties": {
              "created_at": {
                "type": "date"
              },
              "embedding_model": {
                "type": "text"
              },
              "file_id": {
                "type": "text"
              },
              "team_id": {
                "type": "text"
              },
              "updated_at": {
                "type": "date"
              }
            }
          },
          "source": {
            "type": "text",
            "fields": {
              "keyword": {
                "type": "keyword",
                "ignore_above": 256
              }
            }
          }
        }
      },
      "text": {
        "type": "text",
        "fields": {
          "ngram": {
            "type": "text"
          }
        }
      },
      "text_embedding": {
        "type": "dense_vector",
        "index": true,
        "similarity": "cosine"
      }
    }
  }
}


PUT _inference/text_embedding/my-e5-model
{
  "service": "elasticsearch",
  "service_settings": {
    "num_allocations": 1,
    "num_threads": 1,
    "model_id": ".multilingual-e5-small"
  }
}

GET /esre-team-test-chunks/_search
{
  "fields": [
    "text"
  ],
  "knn": {
    "field": "text_embedding",
    "k": 10,
    "num_candidates": 100,
    "query_vector_builder": {
      "text_embedding": {
        "model_id": ".multilingual-e5-small",
        "model_text": "test"
      }
    }
  }
}

GET /esre-team-test-chunks/_search
{
  "fields": [
    "text"
  ],
  "knn": {
    "field": "text_embedding",
    "k": 10,
    "num_candidates": 100,
    "query_vector": [ 1.0, 2.0, 3.0 ]
  }
}

[elasticsearchmachine]

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

[kderusso]

Some whitespace changes auto-generated by IDE - "Hide whitespace" to make this more readable.

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[Samiul-TheSoccerFan]

LGTM

[kderusso]

@elasticmachine update branch

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.15	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 111756

[kderusso]

💚 All backports created successfully

Status	Branch	Result
✅	8.15

Questions ?

Please refer to the Backport tool documentation