Bar charts with null and empty string categories produces odd results

[rlm1023]

Bug Description

Bar charts attempt to stack null and empty string categories on their y-axis, but still create distinct x-axis categories for them, leading to odd charts.

{
  "data": {
    "values": [{"a": null, "b": 28}, {"a": "", "b": 55}, {"a": "C", "b": 43}]
  },
  "mark": "bar",
  "encoding": {
    "x": {"field": "a", "type": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"}
  }
}

Produces:

Repro

Checklist

• I checked for duplicate issues.