[BUG] Opensearch Java client can't index "null" value

What is the bug?

A clear and concise description of the bug.
Opensearch java client can't index null value. When I pass a field with null value, it skips that field altogether. The field is not indexed.

How can one reproduce the bug?

Steps to reproduce the behavior.

{
    "index-1": {
        "mappings": {
            "dynamic": "false",
            "_source": {
                "includes": [
                    "*"
                ]
            },
            "dynamic_templates": [
                {
                    "string_permissions": {
                        "path_match": "perms.*",
                        "mapping": {
                            "type": "keyword"
                        }
                    }
                }
            ],
            "properties": {
                "__createdAt": {
                    "type": "date"
                },
                "__creator": {
                    "type": "keyword"
                },
                "__fromCV": {
                    "type": "long"
                },
                "__id": {
                    "type": "keyword"
                },
                "__modifiedAt": {
                    "type": "date"
                },
                "__modifier": {
                    "type": "keyword"
                },
                "__toCV": {
                    "type": "long"
                },
                "cId": {
                    "type": "keyword"
                },
                "cVersion": {
                    "type": "long"
                },
                "k": {
                    "properties": {
                        "1": {
                            "type": "keyword",
                            "null_value": "NONE"
                        },
                        "2": {
                            "type": "keyword",
                            "null_value": "NONE"
                        },
                        "3": {
                            "type": "keyword",
                            "null_value": "NONE"
                        },
                        "4": {
                            "type": "keyword",
                            "null_value": "NONE"
                        },
                        "5": {
                            "type": "keyword",
                            "null_value": "NONE"
                        }
                    }
                },
                "perms": {
                    "dynamic": "true",
                    "properties": {
                        "acm": {
                            "type": "keyword"
                        },
                        "custom": {
                            "type": "keyword"
                        },
                        "l2cs": {
                            "type": "boolean"
                        },
                        "wipdm": {
                            "type": "keyword"
                        }
                    }
                }
            }
        }
    }
}

When I pass null values for any of the k properties, it skips them. For example, if k.2: null, this value won't be indexed. Other values get properly indexed.

But if I directly index the value k.2: null using OpenSearch dashboard, it gets indexed properly. The same mappings have been used.

What is the expected behavior?

A clear and concise description of what you expected to happen.
Expectation is that even with OpenSearch java client I can index null value

What is your host/environment?

Operating system, version.
macOS & Windows 11

Do you have any screenshots?

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Do you have any additional context?

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No source file or test is named. Start by tracing the Java client's indexing request serialization and compare how a nested null field is represented with the dashboard request. Done means an index request containing k.2: null preserves that field so OpenSearch applies the mapping's null_value behavior.