docs: public static Expression constantNull(Type type) {
      checkArgument(
          type.getSort() == Type.OBJECT || type.getSort() == Type.ARRAY,
          "%s is not a reference type",
          type);
      return new Expression(type, Feature.CHEAP) {
        @Override
        protected void doGen(CodeBuilder mv) {
          mv.visitInsn(Opcodes.ACONST_NULL);
        }
      };
    }