This method is not going to be used anywhere else beside the single use in the constructor in this class. 
The `not_found?` method is used exactly once in every class which inherit from `SystemdUnit`:
```ruby
return nil if unit.not_found?
```
Like here: https://github.com/yast/yast-yast2/blob/master/library/systemd/src/modules/systemd_service.rb#L80