API: ExtensionArray interface method to create an empty / all-NA array for a given dtype

[jorisvandenbossche]

Specific use case comes from #39612 (comment), 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)

[jbrockmendel]

closed by #40602 that implemented EA._empty? #19600 is the analogous issue calling for a EADtype.empy

[jorisvandenbossche]

See also the discussion in #40602: the current interface does not yet provide a way to get an all-NA array, which is half of this issue (if we decide that's useful enough of course).

I think on the other PR, I mentioned the option of adding a keyword to empty() to ask for all-NAs (and if not asking for it, it can still be all NA (as we do for some implementations), but doesn't need to be guaranteed)