typescript-fetch: Fix model date crash

Fix a runtime crash converting to json when a date or datetime is both nullable
AND required. There are 4 cases to account for:

| required | nullable | values |
|----------|----------|--------|
| f        | f        | string OR undefined |
| f        | t        | string OR null OR undefined |
| t        | f        | string |
| t        | t        | string OR null |

And importantly when required and nullable code that would crash on null was
being generated. additionally when required is false and nullable is true we
still want to allow consumers to be able to pass in `undefined` OR `null` and
pass that value to the server. Some servers treat null and undefined differently
for some operations so having that ability is pretty reasonable.

fix: #21820

Author: kradical

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

@@ -141,10 +141,10 @@ export function {{classname}}ToJSONTyped(value?: {{#hasReadOnly}}Omit<{{classnam
         {{^isReadOnly}}
         {{#isPrimitiveType}}
         {{#isDateType}}
-        '{{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)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{#isNullable}}{{#required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{/isNullable}}value['{{name}}'].toISOString().substring(0,10),
         {{/isDateType}}
         {{#isDateTimeType}}
-        '{{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()),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{#isNullable}}{{#required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{/isNullable}}value['{{name}}'].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}},