[weaviate store] Similarity search returns an error if there are no docs in the response

[mdelapenya]

I've playing around with different stores and I noticed the weaviate implementation is behaving differently as others, returning an specific err in the case the datastore returns zero relevant docs.

I checked the other store implementations, and for them it's fine to return an slice of zero elements:

• weaviate:

  langchaingo/vectorstores/weaviate/weaviate.go

  Line 232 in 238d1c7

  return nil, ErrEmptyResponse

• qdrant:

  langchaingo/vectorstores/qdrant/rest.go

  Line 104 in 238d1c7

  docs := make([]schema.Document, len(response.Result))

• chroma:

  langchaingo/vectorstores/chroma/chroma.go

  Line 152 in 238d1c7

  for docsI := range qr.Documents {

• pgvector:

  langchaingo/vectorstores/pgvector/pgvector.go

  Line 342 in 238d1c7

  for rows.Next() {

• milvus:

  langchaingo/vectorstores/milvus/milvus.go

  Line 261 in 238d1c7

  for _, res := range searchResult {

I'd see convenient that all stores define the same expectations.

In the case this is of interest, I can work on the fix.

Thanks!