### 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
final BinaryComparison comparison;
            final Literal bcLiteral;
            final ArithmeticOperation operation;
            final Expression opLeft;
            final Expression opRight;
            final Literal opLiteral;
            OperationSimplifier(BinaryComparison comparison) {
                this.comparison = comparison;
                operation = (ArithmeticOperation) comparison.left();
                bcLiteral = (Literal) comparison.right();
                opLeft = operation.left();
                opRight = operation.right();
                if (opLeft instanceof Literal) {
                    opLiteral = (Literal) opLeft;
                } else if (opRight instanceof Literal) {
                    opLiteral = (Literal) opRight;
                } else {
                    opLiteral = null;