crash on nullish facet data

I think this should throw an error, rather than crashing. Not really a bug since it's not following the general definition of data+channels, but the crashes it produces (in at least two places) show that we're controlling that y is non-null, and then expect facet.data to have a length.

export async function AAA() {
  return Plot.plot({
    facet: {data: null, y: ["a", "b", "c", "d"]},
    marks: [Plot.dotX([1, 2, 4, 5])]
  });
}