Fix uploading of empty files with the urllib based transport.
Fix uploading of empty files with the urllib based transport.

mmap cannot map empty files.

ssorgatem/pulsar,jmchilton/pulsar,jmchilton/lwr,galaxyproject/pulsar,natefoo/pulsar,galaxyproject/pulsar,jmchilton/lwr,ssorgatem/pulsar,jmchilton/pulsar,natefoo/pulsar