Null predicate codes are not currently correctly handled

[gkondas]

Users can currently define plain predicates with code: null and only specify value_min/value_max. This is not in line with the current implementation of ACES and a warning should be raised. This began to be addressed here.

A quick "fix" may be to not add code level filters inside PredicateConfig when code: null. However, due to derived predicates being evaluated AFTER the predicates_df is aggregated by (subject_id,timestamp), " range only" predicates are not compatible with the current order of operations in ACES as discussed here.