[BUG][typescript-fetch] Datetime in classes can't be null

[tikki100]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Currently, when a class is being generated, any dateTime values can not be null. It will replace the value with undefined instead. This means that you can not pass null as a value to any property that is part of a nullable class.

openapi-generator version

7.12.0

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml

Generation Details

cli command:

openapi-generator generate -i openapi.yaml -g typescript-fetch -o src/generated-api --additional-properties=importFileExtension=.js

Or with docker:

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli generate \
    -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/refs/heads/master/modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml \
    -g typescript-fetch \
    -o /local/packages/test/src/generated-api

Possible offending file:

openapi-generator/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

Lines 142 to 147 in a7159f6

	{{#isDateType}}
	'{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString().substring(0,10)),
	{{/isDateType}}
	{{#isDateTimeType}}
	'{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString()),
	{{/isDateTimeType}}

Steps to reproduce

Generate a class with a dateTime property that is nullable:

        shipDate:
          type: string
          format: date-time
          nullable: true

Related issues/PRs

Might be related? #11307

Suggest a fix

Expected result (somewhat like seen here):

        'date_prop': value['dateProp'] === null ? null : ((value['dateProp'] as any)?.toISOString().substring(0,10)),
        'datetime_prop': value['datetimeProp'] === null ? null : ((value['datetimeProp'] as any)?.toISOString()),

Current result (as seen here)

        'date_prop': value['dateProp'] == null ? undefined : ((value['dateProp'] as any).toISOString().substring(0,10)),
        'datetime_prop': value['datetimeProp'] == null ? undefined : ((value['datetimeProp'] as any).toISOString()),