Missing x-axis labels in boxwork() when all data missing in a group.

[kylebaron]

See report from @janellelennie in #100

The box_labels() function needs refactoring to retain x-axis "levels" that contain no non-NA data.

function (df, x, y) {
  .xcol <- rlang::sym(x)
  .ycol <- rlang::sym(y)
  df <- mutate(df, notna = !is.na(!!.ycol))
  df <- group_by(df, !!.xcol)
  .sum <- summarize(df, n = sum(notna), N = n_distinct(ID[notna]))
  .sum <- ungroup(.sum)
  as.data.frame(.sum)
}