docs: public void downgradeOnControlSplit() {
          final int numSlots = getNumSlots();
          for (int i = 0; i < numSlots; ++i) {
              IsNullValue value = getValue(i);
              value = value.downgradeOnControlSplit();
              setValue(i, value);
          }
  
          if (knownValueMap != null) {
              for (Map.Entry<ValueNumber, IsNullValue> entry : knownValueMap.entrySet()) {
                  entry.setValue(entry.getValue().downgradeOnControlSplit());
              }
          }
      }