Avoid "<empty>" labels with qplot()

[yutannihilation]

Fix #4170

devtools::load_all("~/repo/ggplot2")
#> Loading ggplot2
qplot(1:10)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2020-09-06 by the reprex package (v0.3.0)}

[yutannihilation]

Thanks for approving. I'm a bit anxious about the failure on R 3.3 and 3.2, but let's merge this and I'll look into it later...

[clauswilke]

It's a problem with a missing systemfonts package. Not sure what exactly happened there, but it's unrelated to this patch.

[yutannihilation]

@thomasp85
Did systemfonts drop support on R <3.4?

r-lib/systemfonts@f3ba313