[terremark] only parse task result if there is one

lib/fog/terremark/models/task.rb

@@ -18,7 +18,9 @@ module Fog
         new_result = attributes.delete('Result')
         super
         @owner  = connection.parse(new_owner)
-        @result = connection.parse(new_result)
+        if new_result
+          @result = connection.parse(new_result)
+        end
       end
 
       def ready?