code: public boolean isInitialized(Object entity) {
          if (nullable) {
              return true;
          }
          Object variable = getValue(entity);
          return variable != null;
      }
docs: A {@link PlanningVariable#nullable()} value is always considered initialized, but it can still be reinitialized
  with {@link PlanningVariable#reinitializeVariableEntityFilter()}.
  @param entity never null
  @return true if the variable on that entity is initialized