[BUG][TYPESCRIPT FETCH] Nullable dates produce compilation error

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

generated code failed to compile with the following error

Object is possibly 'null'.  TS2531
'documentTypeName': value.documentTypeName,
'documentNo': value.documentNo,
'issueDate': value.issueDate === undefined ? undefined : value.issueDate.toISOString(),
                                                         ^
        };
    }

the generated interface for the value variable is as follows

export interface BillingDocumentReferenceMetadata {
    /**
     * Document type, e.g. invoice, credit note, order, etc.
     * @type {string}
     * @memberof BillingDocumentReferenceMetadata
     */
    documentType?: BillingDocumentReferenceMetadataDocumentTypeEnum;
    /**
     * Document type name, e.g. invoice, credit note, order, etc.
     * @type {string}
     * @memberof BillingDocumentReferenceMetadata
     */
    documentTypeName?: string;
    /**
     * Document number.
     * @type {string}
     * @memberof BillingDocumentReferenceMetadata
     */
    documentNo?: string;
    /**
     * Document issue date (optional).
     * @type {Date}
     * @memberof BillingDocumentReferenceMetadata
     */
    issueDate?: Date | null;
}

export function BillingDocumentReferenceMetadataToJSON(value?: BillingDocumentReferenceMetadata): any {
    if (value === undefined) {
        return undefined;
    }
    return {
        'documentType': value.documentType,
        'documentTypeName': value.documentTypeName,
        'documentNo': value.documentNo,
        'issueDate': value.issueDate === undefined ? undefined : value.issueDate.toISOString(),
    };
}

openapi-generator version

version 4.1.1

OpenAPI declaration file content or url

Part of the swagger.json file that describes the BillingDocumentReferenceMetadata

"BillingDocumentReferenceMetadata": {
    "type": "object",
    "properties": {
        "documentType": {
            "enum": [
                "Invoice",
                "CreditNote",
                "Order",
                "Unspecified"
            ],
            "type": "string",
            "description": "Document type, e.g. invoice, credit note, order, etc."
        },
        "documentTypeName": {
            "type": "string",
            "description": "Document type name, e.g. invoice, credit note, order, etc."
        },
        "documentNo": {
            "type": "string",
            "description": "Document number."
        },
        "issueDate": {
            "type": "string",
            "description": "Document issue date (optional).",
            "format": "date-time",
            "nullable": true
        }
    },
    "additionalProperties": false,
    "description": "Metadata of the reference to another business document."
},

Command line used for generation

npx openapi-generator generate -i http://localhost:5003/swagger/v1/swagger.json -g typescript-fetch -o ./src/ApiClient

Steps to reproduce

Generate a typescript fetch client from a json file that contains a nullable date

Start by generating a typescript-fetch client with the nullable date schema and run TypeScript compilation on the generated output. Inspect the generated BillingDocumentReferenceMetadataToJSON function and verify that the nullable issueDate case compiles successfully while preserving the reported serialization behavior.