firestore/internal/conformance/testdata/set-empty.json

{
  "tests": [
    {
      "description": "set: creating or setting an empty map",
      "set": {
        "docRefPath": "projects/projectID/databases/(default)/documents/C/d",
        "jsonData": "{}",
        "request": {
          "database": "projects/projectID/databases/(default)",
          "writes": [
            {
              "update": {
                "name": "projects/projectID/databases/(default)/documents/C/d",
                "fields": {}
              }
            }
          ]
        }
      }
    }
  ]
}