doc import with empty file errors

[Frando]

$ touch foo
$ iroh doc import foo
Import 1 files totaling 0 B? yes
Error: RpcError(Attempted to insert an empty entry)

Reason is that we currently don't allow to add empty blobs to a document, because it is the same as deleting an entry, because a deleted entry internally is an entry with the content_hash set to the hash of the empty byte string.

This is problematic, because with doc import and doc export a missing (deleted) entry is not the same as an empty entry, because empty files are a totally valid thing.