Empty sets are counted and are unaffected by 'sets' argument

The first example provided with `scale_x_upset()` includes counts for empty sets (3rd bar). I'm can't decide whether this is expected or not but I thought it worth noting in case others come across it and are surprised by it.

``` r
library(ggupset)
library(ggplot2)

ggplot(tidy_movies[1:100, ], aes(x=Genres)) +
geom_bar() +
scale_x_upset(reverse = TRUE, sets=c("Drama", "Action"))
#> Warning: Removed 33 rows containing non-finite values (stat_count).
```


^{Created on 2023-01-06 with [reprex v2.0.2](https://reprex.tidyverse.org)}

Reproduce the reported example with ggupset and ggplot2 using tidy_movies[1:100, ], scale_x_upset(reverse = TRUE, sets=c("Drama", "Action")), and geom_bar(). Inspect how empty sets and the sets argument are handled; done means the behavior is clarified and the unexpected empty-set counts are corrected or explicitly documented.

Written by the indexing model from the issue text.