Empty bins fall back on the first element of their group

[Fil]

For z, stroke and fill. Previously b[0] was undefined, so a spurious undefined value was potentially added to GZ, GF or GS.

cc @yurivish

closes #1775

[Fil]

Thanks! Here's another test:

export async function test2() {
  const fills = {red: d3.interpolateReds, green: d3.interpolateGreens, blue: d3.interpolateBlues};
  const data = []
    .concat(
      d3
        .range(100)
        .map((d) => ({x: d, y: 0.4 + 0.6 * Math.cos(d / 10), z: "red"}))
        .filter((d) => d.y > 0)
    )
    .concat(
      d3
        .range(100)
        .map((d) => ({x: d, y: 0.4 + 0.6 * Math.cos(d / 3), z: "green"}))
        .filter((d) => d.y > 0)
    )
    .concat(
      d3
        .range(100)
        .map((d) => ({x: d, y: 0.4 + 0.6 * Math.cos(d / 7), z: "blue"}))
        .filter((d) => d.y > 0)
    );
  return Plot.areaY(data, {
    x: "x",
    y: "y",
    z: "z",
    fill: (d) => fills[d.z](d.y),
    stroke: "z",
    interval: 1
  }).plot();
}