[BUG] max_score is null when using _score as a primary sort

[gaobinlong]

Describe the bug

When using _score as a primary sort and other field as a second sort, the max_score field in the search result is null, this is not consistent with the case of only sorting by _score:

GET logs-211998/_search
{
  "query": {
    "match": {
      "request": "english/images"
    }
  },
  "sort": [
    {
      "_score": {
        "order": "desc"
      }
    },
    {
      "@timestamp": {
        "order": "desc"
      }
    }
  ]
}

Result:
{
  "took": 2591,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 10000,
      "relation": "gte"
    },
    "max_score": null,
    "hits": [
      {
        "_index": "logs-211998",
        "_id": "OfsV8JUBd7_3mJrIyIT3",
        "_score": 0.47368687,
        "_source": {
          "@timestamp": 895968289,
          "clientip": "88.192.6.0",
          "request": "GET /english/images/team_bu_route_off.gif HTTP/1.0",
          "status": 200,
          "size": 766
        },
        "sort": [
          0.47368687,
          895968289000
        ]
      }...

But when sorting only on _score, the max_score is not null:

GET logs-211998/_search
{
  "query": {
    "match": {
      "request": "english/images"
    }
  },
  "sort": [
    {
      "_score": {
        "order": "desc"
      }
    }
  ]
}

Result:
{
  "took": 136,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 10000,
      "relation": "gte"
    },
    "max_score": 0.47368687,
    "hits": [
      {
        "_index": "logs-211998",
        "_id": "8ccV8JUBd7_3mJrIGguF",
        "_score": 0.47368687,
        "_source": {
          "@timestamp": 895552067,
          "clientip": "242.212.4.0",
          "request": "GET /english/images/news_btn_press_off.gif HTTP/1.0",
          "status": 200,
          "size": 1740
        }
      },

Related component

Search

To Reproduce

As above.

Expected behavior

max_score in the search result should not be null when using _score as a primary sort

Additional Details

OpenSearch 3.0.0 and also the main branch have the same issue.