Given a list of user IDs, fetch each user from the DummyJSON API and build a map of user first names to their city. The API endpoint is https://dummyjson.com/users/{id} and the response includes a "firstName" field at the top level and an "address" object containing a "city" field. Return an empty map for null or empty input. If a request fails, skip that user and continue with the next one.

The DummyJSON user response structure:

{
  "id": 1,
  "firstName": "Emily",
  "address": {
    "city": "Phoenix"
  }
}

Examples: