@Test
public void is_null_predicate() {

  FunctionExpression expression = dsl.isnull(DSL.literal(1));
  assertEquals(BOOLEAN, expression.type());
  assertEquals(LITERAL_FALSE, expression.valueOf(valueEnv()));
  expression = dsl.isnull(DSL.literal(ExprNullValue.of()));
  assertEquals(BOOLEAN, expression.type());
  assertEquals(LITERAL_TRUE, expression.valueOf(valueEnv()));
}