geom_sina omits data when input/facet has one row

[glocke01]

related https://github.com/thomasp85/ggforce/issues/51

reprex:

x <- tibble(grp=c("a", "b", "b", "b", "b"), y=1:5)
ggplot(x, aes(x=grp, y=y)) + geom_sina() # works
x %>% head(n=1) %>% ggplot(aes(x=grp, y=y)) + geom_sina() ## no dots plotted, gives warning

Warning message: Computation failed in stat_sina(): binwidth must be positive

granted that example is silly, but here's how I found the problem

ggplot(x, aes(x=grp, y=y)) + geom_sina() + facet_wrap(~grp, ncol=2) # no dot for grp==a, gives warning

Warning message: Removed 1 rows containing missing values (geom_point).