fix(typescript-fetch): Use null as a value when the date value is nullable (OpenAPITools#21133)

* fix use null when available for dates

* generated typescript-fetch-default-v3.0 sample

* ran all typescript examples

Author: tikki100

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

@@ -140,10 +140,10 @@ export function {{classname}}ToJSONTyped(value?: {{#hasReadOnly}}Omit<{{classnam
         {{^isReadOnly}}
         {{#isPrimitiveType}}
         {{#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)),
+        '{{baseName}}': {{^required}}{{#isNullable}}value['{{name}}'] === null ? null : {{/isNullable}}{{^isNullable}}value['{{name}}'] == null ? undefined : {{/isNullable}}{{/required}}((value['{{name}}']{{#isNullable}} as any{{/isNullable}}){{^required}}{{#isNullable}}?{{/isNullable}}{{/required}}.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()),
+        '{{baseName}}': {{^required}}{{#isNullable}}value['{{name}}'] === null ? null : {{/isNullable}}{{^isNullable}}value['{{name}}'] == null ? undefined : {{/isNullable}}{{/required}}((value['{{name}}']{{#isNullable}} as any{{/isNullable}}){{^required}}{{#isNullable}}?{{/isNullable}}{{/required}}.toISOString()),
         {{/isDateTimeType}}
         {{#isArray}}
         '{{baseName}}': {{#uniqueItems}}{{^required}}value['{{name}}'] == null ? undefined : {{/required}}{{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}Array.from(value['{{name}}'] as Set<any>){{/uniqueItems}}{{^uniqueItems}}value['{{name}}']{{/uniqueItems}},

samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts

@@ -143,8 +143,8 @@ export function NullableClassToJSONTyped(value?: NullableClass | null, ignoreDis
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        '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()),
+        '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()),
         'array_nullable_prop': value['arrayNullableProp'],
         'array_and_items_nullable_prop': value['arrayAndItemsNullableProp'],
         'array_items_nullable': value['arrayItemsNullable'],

samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts

@@ -143,8 +143,8 @@ export function NullableClassToJSONTyped(value?: NullableClass | null, ignoreDis
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        '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()),
+        '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()),
         'array_nullable_prop': value['arrayNullableProp'],
         'array_and_items_nullable_prop': value['arrayAndItemsNullableProp'],
         'array_items_nullable': value['arrayItemsNullable'],