### 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/esql/src/main/java/org/elasticsearch/xpack/esql/optimizer/LocalLogicalPlanOptimizer.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.esql.optimizer;
import org.elasticsearch.xpack.esql.plan.logical.Eval;
import org.elasticsearch.xpack.esql.plan.logical.TopN;
import org.elasticsearch.xpack.esql.stats.SearchStats;
import org.elasticsearch.xpack.ql.expression.Alias;
import org.elasticsearch.xpack.ql.expression.FieldAttribute;
import org.elasticsearch.xpack.ql.expression.Literal;
import org.elasticsearch.xpack.ql.expression.NamedExpression;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules;
import org.elasticsearch.xpack.ql.plan.logical.Aggregate;
import org.elasticsearch.xpack.ql.plan.logical.EsRelation;
import org.elasticsearch.xpack.ql.plan.logical.Limit;
import org.elasticsearch.xpack.ql.plan.logical.LogicalPlan;
import org.elasticsearch.xpack.ql.plan.logical.OrderBy;
import org.elasticsearch.xpack.ql.plan.logical.Project;
import org.elasticsearch.xpack.ql.rule.ParameterizedRule;
import org.elasticsearch.xpack.ql.rule.ParameterizedRuleExecutor;
import java.util.ArrayList;
import java.util.List;
import static org.elasticsearch.xpack.ql.optimizer.OptimizerRules.TransformDirection.UP;
public class LocalLogicalPlanOptimizer extends ParameterizedRuleExecutor<LogicalPlan, LocalLogicalOptimizerContext> {