The learned values (impute_numeric_values, impute_level_values,
scale_centers, scale_coefficients, one_hot_levels,
factor2integer_levels) are data_dependent settable inputs: a
user-authored config typically carries none, and the setup_* defaults leave
them unset. A config that does carry them -- a fitted preprocessor written
back out -- restores them, which is why they are serialized at all (see
"Property roles" in 010_Props.R).