Skip keywords with empty string arguments

This commit fixes issue #57.

Since all config options consist of both a keyword and an argument, skip adding any keywords to ssh_config or sshd_config where the argument is an empty string.

One remaining issue with this is that with config hashes, one might end up with only the hash title and no options. For instance, I got an ssh_config with only the line "Host *" when I disabled the SendEnv option. The ssh client didn't complain, however, so hopefully it doesn't matter.