bugfix: don't set undef values in ssh_config

Analogous to the check already done here: https://github.com/saz/puppet-ssh/blob/master/templates/sshd_config.erb#L44
Don't set values in the client config, that are configured to be empty (i.e. `undef` in puppet).

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>