Fixes for LLADDR="00:00:00:00:00:00"

Should address at least: bnc#889287, bnc#898620


Actually it can be `nil` and that case is very relevant here! http://www.rubydoc.info/github/yast/yast-yast2/Yast/FileUtilsClass#IsDirectory-instance_method

- It does not exist → `nil`
- It does exist, not a directory → `false`
- It does exist, is a directory → `true`