ignore empty facets, take 2

[Fil]

fixes #97

supersedes #98

test 1:

https://observablehq.com/d/2c7e3835c809e7a5

test 2:

https://observablehq.com/@data-workflows/geofacet

[Fil]

Two things worry me a bit in this PR:

• the single path means that we can't select a facet and move it around (in Illustrator); it seems to me that this would be better in the other approach (faceted grid #98)
• the code is a bit weird: we send a filtered domain of fx in index, or undefined if there is only 1 facet axis, but when index is undefined we used fy.domain

[Fil]

I've rebased this PR. Moving to draft while I work on #277.

---

I've also tried to add axes on (x,y) facets that have no left/bottom neighbor. It sort of works

but it would create two new issues:

1. Where should we put the axis label?

2. The axes tend to collide

[Fil]

superseded by #332