bugfix: don't set 'undef' values in ssh client config

This is a workaround until saz/ssh is updated to include this fix upstream. Potentially it may stay in longer, depending on what is the lesser evil: requiring puppetlabs stdlib to be at least v4.2 or requiring saz/ssh to be a minimum of the next version.

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>