using empty character for indicating that generator expects no value

lib/refrepo/gen/oar-properties.rb

@@ -959,7 +959,7 @@ def do_diff(options, generated_hierarchy, refrepo_properties)
               {:cpu => "cpu", :core => "core", :cpuset => "cpuset", :gpu => "gpu", :gpudevice => "gpudevice"}.each do |key, value|
                 if row[key].to_s != corresponding_resource[0][value].to_s and not (key == :gpu and row[key].nil? and corresponding_resource[0][value] == 0)
                   expected_value = row[key]
-                  if expected_value == ""
+                  if expected_value == "" or expected_value.nil?
                     expected_value = "ø"
                   end
                   fix_cmd = <<-TXT