Empty subtypes of Index return their type, rather than Index
Resolves #10596 

Prior was in #10599 FYI

I think this has a PyTables error which I need to resolve

did you resubmit for some reason?

#10599 was merged (I merged it directly rather than thru github). I just closed the issued (forgot to before)
 @jreback Great - didn't know it was merged. Cheers!
 no thank you!
