block_types: handle data version for an empty iptrs list (#725)
block_types: handle data version for an empty iptrs list (#725)

A list of IPtrs in an indirect block can be empty if the file has been
truncated to 0 bytes, since we don't currently remove the level of
indirection in that case.

Issue: KBFS-1924
keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client