Cloudless sky in synop message?
Yes, in your example, the group should have been omitted. From the WMO Publication No. 306 - Manual On Codes - Volume I.1 - Part A, Section A, FM 12 SYNOP:


  12.2.7 Group 8NhCLCMCH
  
  12.2.7.1 This group shall be omitted in the following cases:
  
  (a) When there are no clouds (N = 0);
  
  (b) When the sky is obscured by fog and/or other meteorological phenomena (N = 9);
  
  (c) When the cloud cover is indiscernible for reasons other than (b) above, or observation is not made (N = /).


metaf2xml tries to stick to the Manual but tolerates deviations, so the group is recognized but marked as invalid.