(#20893) Empty output from facts.d scripts crashes facter

Without this patch facter will crash with 'Error: undefined method `each_line' for nil:NilClass' if a script in /etc/facter/facts.d has no output.

The problem is caused because Facter::Util::Resolution.exec can return nil. The calling function doesn't check for this and does an each_line on nil. This patch adds the check for nil.