mark.invalid set the invalid points on y channel at the top

[yhoonkim]

Here is the repro.

{
  "data": {
    "values": [
      {"color": "Blue", "y": 10},
      {"color": "Green", "y": null},
      {"color": "Red", "y": 20}
    ]
  },
  "encoding": {
    "x": {"field": "color", "type": "nominal", "title": "color"},
    "y": {"field": "y", "type": "quantitative"}
  },
  "mark": {"type": "line", "tooltip": true},
  "config": {"mark": {"invalid": null}}
}

It is supposed to place "Green" value at the bottom (the 0 value position (yScale(0)) not pure 0 position)

[kanitw]

will be fixed in #9342!