### Description

The IS NULL/IS NOT NULL only care about the nullability not the value of an expression.
Given the expression x + 1 / 2, the actual result does not matter only if it's null or not - that is, it only matters if x is null or not.
So x + 1 / 2 IS NULL becomes x IS NULL - which can be opportunistically pushed down or evaluated.
x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/optimizer/OptimizerRules.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.ql.optimizer;
import org.elasticsearch.common.util.set.Sets;
import org.elasticsearch.xpack.ql.expression.Alias;
import org.elasticsearch.xpack.ql.expression.Expression;
import org.elasticsearch.xpack.ql.expression.Expressions;
import org.elasticsearch.xpack.ql.expression.Literal;
import org.elasticsearch.xpack.ql.expression.Nullability;
import org.elasticsearch.xpack.ql.expression.Order;
import org.elasticsearch.xpack.ql.expression.function.Function;
import org.elasticsearch.xpack.ql.expression.function.Functions;
import org.elasticsearch.xpack.ql.expression.function.scalar.ScalarFunction;
import org.elasticsearch.xpack.ql.expression.function.scalar.SurrogateFunction;
import org.elasticsearch.xpack.ql.expression.predicate.BinaryOperator;
import org.elasticsearch.xpack.ql.expression.predicate.BinaryPredicate;
import org.elasticsearch.xpack.ql.expression.predicate.Negatable;
import org.elasticsearch.xpack.ql.expression.predicate.Predicates;
import org.elasticsearch.xpack.ql.expression.predicate.Range;
import org.elasticsearch.xpack.ql.expression.predicate.logical.And;