Wish: `FeatureEffect` to gain an option for showing NA for empty cells

`ALEPlot::ALEPlot` has an argument `NA.plot` that is used for 2D plots. This argument is in my view important where there are no data in some regions of the experimental space (for example in the lower right and upper left areas in the first example in [https://christophm.github.io/interpretable-ml-book/ale.html#examples-2](https://christophm.github.io/interpretable-ml-book/ale.html#examples-2)): it should be set to `FALSE` in such cases. As far as I can see, this is currently not possible in package **iml**; I suppose that `iml::FeatureEffect$new` would be the right place for such an argument?

Best, Ulrike