### 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/test/java/org/elasticsearch/xpack/esql/optimizer/LocalLogicalPlanOptimizerTests.java
import org.elasticsearch.xpack.esql.parser.EsqlParser;
import org.elasticsearch.xpack.esql.plan.logical.Eval;
import org.elasticsearch.xpack.esql.plan.logical.local.LocalRelation;
import org.elasticsearch.xpack.esql.plan.logical.local.LocalSupplier;
import org.elasticsearch.xpack.esql.stats.SearchStats;
import org.elasticsearch.xpack.ql.expression.Alias;
import org.elasticsearch.xpack.ql.expression.Expressions;
import org.elasticsearch.xpack.ql.expression.Literal;
import org.elasticsearch.xpack.ql.expression.ReferenceAttribute;
import org.elasticsearch.xpack.ql.index.EsIndex;
import org.elasticsearch.xpack.ql.index.IndexResolution;
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.Project;
import org.elasticsearch.xpack.ql.type.DataTypes;
import org.elasticsearch.xpack.ql.type.EsField;
import org.junit.BeforeClass;
import java.util.List;
import java.util.Map;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.L;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.TEST_VERIFIER;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.as;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.loadMapping;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.statsForMissingField;
import static org.elasticsearch.xpack.esql.EsqlTestUtils.withDefaultLimitWarning;
import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.nullValue;
public class LocalLogicalPlanOptimizerTests extends ESTestCase {