Fix the discrete scale when all values are continuous

[yutannihilation]

Fix #3463

After #3426, the following fails because scale$get_limits() returns NULL. This PR fixes this by reverting if branches to the previous code while the functional limits are still accepted.

library(ggplot2)

df <- data.frame(x = c(0, 2, 4), y = 1:3, stringsAsFactors = FALSE)

ggplot(df, aes(x, y)) +
  geom_point() +
  scale_x_discrete()
#> Warning in structure(in_domain, pos = match(in_domain, breaks)): Calling 'structure(NULL, *)' is deprecated, as NULL cannot have attributes.
#>   Consider 'structure(list(), *)' instead.

#> Warning in structure(in_domain, pos = match(in_domain, breaks)): Calling 'structure(NULL, *)' is deprecated, as NULL cannot have attributes.
#>   Consider 'structure(list(), *)' instead.
#> Error in UseMethod("rescale"): no applicable method for 'rescale' applied to an object of class "list"

^{Created on 2019-07-30 by the reprex package (v0.3.0)}

(As this bug only exists in the dev version, no NEWS bullet is needed here)

[thomasp85]

LGTM

[yutannihilation]

Thanks!

[lock]

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