Remove validate_not_empty from validate_multi_ssh

This patch removes the validate_not_empty call from the
validate_multi_ssh validator. As a result, validate_multi_ssh will
succeed for null parameter. The validate_not_empty validator should be
added to a parameters list of validators as needed. This allows
validation to be more flexible.

Change-Id: I4668f9d4fd8f57f78183ee24842cbfbb6ee979a5