Fix: bulk response _source.

[dblock]

Description

Two bugs fixed here.

1. The value of _source is a document, so it is additionalProperties: true, not a set of objects.

   - { doc: { pages: 376 }, _source: true }
   - { update: { _index: books, _id: book_1392214 } }
   

2. In a scripted_upsert a document may not be found, therefore not returning _source. There was an attempt to fix this in Made the InlineGet source field optional. #351, this PR replaces that one.

   - { update: { _index: books, _id: does_not_exist } }
   - { script: { source: 'ctx.op = "none";' }, scripted_upsert: true, upsert: { pages: 375 } }
   

Full response.

{
      "update": {
        "_index": "books",
        "_id": "book_1392214",
        "_version": 2,
        "result": "noop",
        "_shards": {
          "total": 2,
          "successful": 1,
          "failed": 0
        },
        "_seq_no": 1,
        "_primary_term": 1,
        "get": {
          "_seq_no": 1,
          "_primary_term": 1,
          "found": true,
          "_source": {
            "author": "Harper Lee",
            "title": "To Kill a Mockingbird",
            "year": 1960,
            "pages": 376
          }
        },
        "status": 200
      }
    }

Without a _source.

{
      "update": {
        "_index": "books",
        "_id": "does_not_exist",
        "_version": -1,
        "result": "noop",
        "_shards": {
          "total": 2,
          "successful": 1,
          "failed": 0
        },
        "get": {
          "found": false
        },
        "status": 200
      }
    }

Issues Resolved

Closes #350.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Changes Analysis

Commit SHA: d249f30
Comparing To SHA: 25521f1

API Changes

Summary

└─┬Components
  ├─┬_common:InlineGet
  │ └──[➖] required (26575:11)❌ 
  └─┬_common:InlineGetDictUserDefined
    ├──[➖] required (26597:11)❌ 
    └─┬_source
      └──[🔀] additionalProperties (26592:33)❌ 

Document Element	Total Changes	Breaking Changes
components	3	3

• ❌ BREAKING Changes: 3 out of 3
• Modifications: 1
• Removals: 2
• Breaking Removals: 2
• Breaking Modifications: 1

Report

The full API changes report is available at: https://github.com/opensearch-project/opensearch-api-specification/actions/runs/9785125147/artifacts/1665784777

API Coverage

	Before	After	Δ
Covered (%)	483 (47.31 %)	483 (47.31 %)	0 (0 %)
Uncovered (%)	538 (52.69 %)	538 (52.69 %)	0 (0 %)
Unknown	24	24	0