Boxplot disappear when there is one value per group

[kanitw]

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "A simple bar chart with embedded data.",
  "data": {
    "values": [
      {"a": "A", "b": 28}, {"a": "A", "b": 30}, {"a": "B", "b": 55}
    ]
  },
  "mark": "boxplot",
  "encoding": {
    "x": {"field": "a", "type": "nominal", "axis": {"labelAngle": 0}},
    "y": {"field": "b", "type": "quantitative"}
  }
}

Nothing shows up for B.

This is similiar to #3488, but is the same problem for boxplot.

[joelostblom]

There is a somewhat similar issue in #8126 where the median boxplot lines disappears with many identical values, just in case anything here could be applied to fix that too.