Fix a bug about strip.placement in facet_wrap()

[yutannihilation]

Fix #3045

When theme$strip.placement is NULL, it should be treated as "inside". It might be better if that default value is set when theme is created, but I just want to fix the apparent inconsistency for now.

library(ggplot2)

ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  facet_wrap(~cyl, ncol = 2L, strip.position = "bottom") +
  theme_void()

^{Created on 2018-12-27 by the reprex package (v0.2.1)}

[yutannihilation]

@clauswilke Could you review this when you have a chance? I expect #2784 will fix this kind of problems fundamentally, so I'm not fully sure we need this particular fix or not. If you think this is not necessary, I'll close this.

[clauswilke]

Could you ask @thomasp85 for a review? I am currently traveling, and I don't know this code very well.

[yutannihilation]

@clauswilke Oh, sorry. Sure. Have a nice trip.

[thomasp85]

LGTM

[yutannihilation]

Thanks!

[lock]

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