fastcdc fails to chunk empty files because of mmap

[stoarca]

The following throws an error if abs_file is empty (such as a python __init__ file)

    for chunk in fastcdc(abs_file, 2**11, 2**14, 2**15, hf=blake3.blake3, fat=True):

It fails with the following error:

ValueError: cannot mmap an empty file

This worked correctly before mmap was implemented.

My use case is that I need to sync an entire directory to another server and I use fastcdc to reduce the transfer size. I cannot control which files the user adds to the transfer, so sometimes they will be empty files. But the files themselves need to be preserved.