set_attribute_value_py turned every Python None into blank{}, so
createIfcSIUnit(None, ...) stored a null in the derived Dimensions slot.
The STEP output still shows "*" and the Python accessor still resolves
the value, but validate() reads the raw slot and reports "Attribute is
derived in subtype" for every such instance. 0.8.5 accepted the same
call and kept the derived marker. Mirror populate_derived_() and store
derived{} when the target slot is derived.
Petru Conduraru
481becfc05