Scatterplot with Null Values in Grey

{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.json",
  "data": {
    "url": "data/movies.json"
  },
  "mark": "point",
  "encoding": {
    "x": {
      "field": "IMDB_Rating",
      "type": "quantitative"
    },
    "y": {
      "field": "Rotten_Tomatoes_Rating",
      "type": "quantitative"
    },
    "color": {
      "condition": {
        "test": "datum.IMDB_Rating === null || datum.Rotten_Tomatoes_Rating === null",
        "value": "#aaa"
      }
    }
  },
  "config": {
    "invalidValues": null
  }
}