Unbreak Config on empty options

This prevents the following issue:

Traceback (most recent call last):
  File "/usr/local/bin/s3cmd", line 2521, in <module>
    rc = main()
  File "/usr/local/bin/s3cmd", line 2261, in main
    cfg = Config(options.config, options.access_key, options.secret_key)
  File "/usr/local/lib/python2.7/dist-packages/S3/Config.py", line 126, in __init__
    self.read_config_file(configfile)
  File "/usr/local/lib/python2.7/dist-packages/S3/Config.py", line 226, in read_config_file
    self.update_option(option, cp.get(option).strip())
AttributeError: 'NoneType' object has no attribute 'strip'