do not raise exception if grub.cfg is not available

src/lib/bootloader/sections.rb

@@ -36,7 +36,7 @@ def default=(value)
       log.info "set new default to '#{value.inspect}'"
 
       # empty value mean no default specified
-      raise "Unknown value #{value.inspect}" if !all.include?(value) && !value.empty?
+      raise "Unknown value #{value.inspect}" if !all.empty? && !all.include?(value) && !value.empty?
 
       @default = value
     end