ssh_config: Don't populate options that are set to undef

Undef values are translated into nil in erb templates in newer
versions of Puppet. This commit is a direct port of commit
86914f4aebcaed97c512cde7acafc52771ef83c4, which made the same change for
sshd_config.erb.

Signed-off-by: Andrew Gunnerson <andrew.gunnerson@us.ibm.com>