Ingest pipeline _simulate request with empty docs array never returns a response

[dolaru]

Elasticsearch version (bin/elasticsearch --version): 7.6.0

Description of the problem including expected versus actual behavior:
When sending an ingest pipeline _simulate request, If the value of docs is an empty array, a response is never returned.

I would've expected the request to either immediately fail and ask for the docs array to have at least 1 document, or have the request succeed and return:

{
  "docs" : []
}

Steps to reproduce:

1. Send POST _ingest/pipeline/_simulate with the following payload:

{
  "pipeline": {
    "processors": []
  },
  "docs": []
}

2. Notice that a response is never returned.

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Ingest)

[danhermann]

Fixed by #52937