(#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