Keep missingness of qplot() to label axis nicely

[yutannihilation]

Closes #3467.

In #2936, I added .ignore_empty = "all" to rlang::enquos() in aes() and in qplot() to remove missing x and y automatically. But, as qplot() generates labels directly from the input, missingness should be kept there. This PR reverts #2936's changes about qplot().

library(ggplot2)

set.seed(2009)
x <- rnorm(100)
qplot(x, geom = "histogram")
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

^{Created on 2019-08-06 by the reprex package (v0.3.0)}

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