Fix missing as group in table_one

[jkrumbiegel]

Fixes #45 and also fixes #46 because the latter happened to come up while fixing the former.

Now you can do:

data = (;
    value = 1:6,
    group1 = ["A", missing, "A", "B", "B", missing],
    group2 = ["D", missing, "D", "D", missing, missing],
)

t = table_one(data, [:value], groupby = [:group1, :group2], group_totals = [:group2])