[BUG]: Defaults not applied #24
Description
Description of the bug
When a value is not set in the config.yml the default is not applied but instead the value null is used.
Steps To Reproduce
Just leave a value out in the config. For example leave out output.iso_filename and the iso file will be generated into a file called null
Additional Information
#23 tries to solve this. It works for regular values, but not for lists as required for the package list of software to be installed.