expand_limits throws warnings

[thomasp85]

After the changes in befc4d0 where unused aesthetics throw a warning you now get a warning when using expand_limits() (that calls geom_blank() under the hood)

ggplot(mtcars) + 
  geom_point(aes(mpg, cyl)) + 
  expand_limits(x = c(-10, NA))

[thomasp85]

@hadley can you look at this since you made the change?