(#22622) External output defaults to empty string

 - Previously, if an external fact generated empty output, parse would
   be called with a nil, which cannot have each_line called on it, and
   would cause Facter to crash.  This sets a reasonable default for
   external command output to the empty string
Merge pull request #567 from Iristyle/ticket/stable/22622-external-facts-without-output-cause-failures

(#22622) External output defaults to empty string