Account for empty archives in hash calculations

This will send back a response from glacier stating
that we can't have a content-length of 0.

I think this makes the most sense, especially if empty
archives ever are allowed, then we don't have to change
anything on our end.
Merge branch 'glacier-tree-hash' into develop

Fixes #1083.

* glacier-tree-hash:
  Account for empty archives in hash calculations