Do not switch state for zero length chunk
Do not switch state for zero length chunk

(cherry picked from commit f565154d5691d5422269cc34d0a52a0d2c813068)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend