CuraEngine from command line fails to accept default_value from non-leaf config json nodes

[richfelker]

Running CuraEngine with the fdmprinter.def.json file from Cura repo is expected to work. However, it errors out with

Trying to retrieve setting with no value given: roofing_layer_count

despite the setting being present in the config json. The root cause seems to be the addition of child config nodes; without them (or with them moved out of it), fetching the value works.