Empty files are not downloaded

When downloading files from blob storage, only files that are not empty are downloaded.
If given the exact path to the empty file, blobporter detects it but doesn't download.

Example information when downloading single empty file:
```
./blobporter -a somestorageaccount -k somekey -c "somecontainer" -n "path/to/files/emptyfile" -t blob-file
BlobPorter
Copyright (c) Microsoft Corporation.
Version: 0.6.15
---------------

Files to Transfer (blob-file) :
Source: some.url.windows.net/somecontainer/path/to/files/emptyfile Size:0

The data transfer took 293.955µs to run.
Throughput: 0.00 MB/s (0.00 Mb/s)
Configuration: R=40, W=60, DataSize=0B (0), Blocks=0
Cumulative Writes Duration: Total=0s, Avg Per Worker=0s
Retries: Avg=NaN Total=0
```