[BUG]: Faceting drops entries with null values

[jamesscottbrown]

This Observable Plot example renders like this:

It translates to SveltePlot as:

<Plot grid="true" marginRight={60} facet={{ label: null }}>
    <Frame />
    <Dot
        data={penguins}
        x="culmen_length_mm"
        y="culmen_depth_mm"
        fx="sex"
        fy="species" />
</Plot>

which then renders as:

The 11 penguins with sex = null have been dropped, removing a column from the matrix of plots.

(this is one of the issues surfaced by #72)