> curl --data @- -X PATCH --dump - http://localhost:8529/_api/graph/graph1/vertex/v1
{"optional2" : "vertexPatch2"}

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
etag: 24332193

{
  "vertex": {
    "_id": "v/v1",
    "_rev": "24332193",
    "_key": "v1",
    "optional1": "val2",
    "optional2": "vertexPatch2"
  },
  "error": false,
  "code": 200
}

> curl --data @- -X PATCH --dump - http://localhost:8529/_api/graph/graph1/vertex/v1?keepNull=false
{"optional2" : null}

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
etag: 24332199

{
  "vertex": {
    "_id": "v/v1",
    "_rev": "24332199",
    "_key": "v1",
    "optional1": "val2"
  },
  "error": false,
  "code": 200
}