### 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/test/java/org/elasticsearch/xpack/ql/optimizer/OptimizerRulesTests.java
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.GreaterThan;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.GreaterThanOrEqual;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.In;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.LessThan;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.LessThanOrEqual;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.NotEquals;
import org.elasticsearch.xpack.ql.expression.predicate.operator.comparison.NullEquals;
import org.elasticsearch.xpack.ql.expression.predicate.regex.Like;
import org.elasticsearch.xpack.ql.expression.predicate.regex.LikePattern;
import org.elasticsearch.xpack.ql.expression.predicate.regex.RLike;
import org.elasticsearch.xpack.ql.expression.predicate.regex.RLikePattern;
import org.elasticsearch.xpack.ql.expression.predicate.regex.WildcardLike;
import org.elasticsearch.xpack.ql.expression.predicate.regex.WildcardPattern;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.BinaryComparisonSimplification;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.BooleanFunctionEqualsElimination;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.BooleanSimplification;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.CombineBinaryComparisons;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.ConstantFolding;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.FoldNull;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.LiteralsOnTheRight;
import org.elasticsearch.xpack.ql.optimizer.OptimizerRules.PropagateEquals;
import org.elasticsearch.xpack.ql.plan.logical.Aggregate;
import org.elasticsearch.xpack.ql.plan.logical.EsRelation;
import org.elasticsearch.xpack.ql.plan.logical.Filter;
import org.elasticsearch.xpack.ql.plan.logical.LogicalPlan;
import org.elasticsearch.xpack.ql.plan.logical.Project;
import org.elasticsearch.xpack.ql.tree.NodeInfo;
import org.elasticsearch.xpack.ql.tree.Source;
import org.elasticsearch.xpack.ql.type.DataType;
import org.elasticsearch.xpack.ql.type.DataTypes;