Missing default for sentinel setting, causes empty file resource name

Hello! This PR addresses a missing default, that if omitted by downstream causes a file resource with an empty string as the name:

```
      file current['logfile'] do
        owner current['user']
        group current['group']
        mode '0644'
        backup false
        action :touch
        only_if { current['logfile'] && current['logfile'] != 'stdout' }
      end
```

Due to the way chef parses resources, `current['logfile']` has to exist to create the resource, which all happens long before ``` only_if { current['logfile'] && current['logfile'] != 'stdout' }``` is evaluated.