guard against passing zero byte data into compressor

it appears that on occasion either Fastly pushes zero length files into our bucket, or DreamHost Objects fails to store any data in keys.

regardless the outcome is the same:

```
+ export PYPI_LOG_DATE=2014-05-02
+ PYPI_LOG_DATE=2014-05-02
+ /opt/pypi-cdn-log-archiver/env/bin/pypi-cdn-log-archiver-wrapper.sh
Traceback (most recent call last):
  File "/opt/pypi-cdn-log-archiver/env/bin/pypi-cdn-log-archiver", line 130, in <module>
    sys.exit(main())
  File "/opt/pypi-cdn-log-archiver/env/bin/pypi-cdn-log-archiver", line 116, in main
    target_date.strftime('%Y-%m-%d'))
  File "/opt/pypi-cdn-log-archiver/env/bin/pypi-cdn-log-archiver", line 55, in archive_logs
    compress_logs(archive_file, temp_dir)
  File "/opt/pypi-cdn-log-archiver/env/bin/pypi-cdn-log-archiver", line 42, in compress_logs
    archive.write(log.read())
  File "/opt/pypi-cdn-log-archiver/env/lib/python2.7/site-packages/backports/lzma/__init__.py", line 327, in write
    compressed = self._compressor.compress(data)
_lzma.LZMAError: Insufficient buffer space
```

apparently zero bytes can not be compressed :sheep: