API: ExtensionArray interface method to create an empty / all-NA array for a given dtype
Specific use case comes from https://github.com/pandas-dev/pandas/pull/39612#discussion_r570932180, but in general it can sometimes be useful to be able to create an empty or all-NA array in an efficient way. 

Since the ExtensionDtype/Array knows best how to do this efficiently, we can add a method to the extension array interface for this.

One question, though: I mentioned above "empty or all-NA", but this is not the same of course. An actually uninitialized array (`np.empty(n, dtype)`) can still be faster to create as an all-NA array (`np.full(n, NA, dtype)`, or variant with the appropriate NA-sentinel).

But maybe we don't care much about this distinction, and just a version to create all-NA is fine? (or another constant fill_value)

