BambuStudio and its forks save a nozzle variant that matches the parent
preset as "nil", including in keys OrcaSlicer can't leave empty, such as
retraction, z-hop and nozzle temperature. Reading one threw, which ended
the rest of the settings file: a project kept only the keys before it
alphabetically, and an embedded or user preset was dropped.
load_from_json now reads such a slot as not set when the caller opts in
(project settings, embedded presets, user presets). If every slot is nil,
the key is left out. Otherwise the slot holds the option default and is
recorded, and update_diff_values_to_child_config gives it the parent
preset's value. Other loaders still reject nil.
Hanif Koh
5ecb596b1c