(#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
(#22622) Resolution.exec returns '' instead of nil

 - Digging up existing calls to each_line yielded some unusual
   functionality where empty strings from output were returned as nil
   instead of ""
 - Uptime appeared to be the only existing fact dependent on this
   behavior and code / tests were updated accordingly
 - With Resolution.exec guaranteed to always return an empty string,
   some extraneous null checks could be stripped