Corner case fixes for a single item UpSetR

[zeehio]

This commit provides some fixes to cover a corner case where there is only one element to intersect.

While its utility is clearly limited, I was using UpSetR on a pipeline with an arbitrary number of items, and before this commit UpSetR was failing for this (trivial) case.

library(UpSetR)
data <- fromList(list(a = c("b", "c")))
upset(data, nsets=6, keep.order = TRUE, order.by = "freq")

Now the output is trivial, but as expected: