[REQ] [typescript-fetch] Add unit test for required nullable datetimes

New test for required nullable datetimes

A bug was recently introduced where required nullable datetimes would cause an issue (runtime crash) for clients when generating the code.

Describe the solution you'd like

A new test could be created that could catch this in the future if any other changes are made to the template.

Additional context

OpenAPI Specification:

{
  "type": "object",
  "properties": {
    "startedAt": {
      "type": "string",
      "format": "date-time",
      "nullable": true
    }
  },
  "required": [
    "startedAt"
  ]

See issue #21820

Start by locating the existing typescript-fetch generator tests and the template or test entry point for generated clients. Use the provided OpenAPI schema with a required nullable date-time property, and consider the work done when generation and the generated client complete without the runtime crash described in issue #21820.