Add a check for zero sized objects and skip them.

[phirestalker]

For the bucket I was dumping, I was getting errors about files being skipped because they already existed. I checked and some known directories were showing up as regular files. I added a check for zero size objects and skip them with a notice (only if verbose) that they were skipped because they are likely directories. After the change, all files are now downloaded without issue.

fixes: #125