### 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
public void testCombineBinaryComparisonsDisjunctionLowerBound() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThan gt1 = greaterThanOf(fa, ONE);
        GreaterThan gt2 = greaterThanOf(fa, TWO);
        GreaterThan gt3 = greaterThanOf(fa, THREE);
        Or or = new Or(EMPTY, gt1, new Or(EMPTY, gt2, gt3));
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(GreaterThan.class, exp.getClass());
        GreaterThan gt = (GreaterThan) exp;
        assertEquals(ONE, gt.right());
    }
    
    public void testCombineBinaryComparisonsDisjunctionIncludeLowerBounds() {
        FieldAttribute fa = getFieldAttribute();
        GreaterThan gt1 = greaterThanOf(fa, ONE);
        GreaterThan gt2 = greaterThanOf(fa, TWO);
        GreaterThanOrEqual gte3 = greaterThanOrEqualOf(fa, THREE);
        Or or = new Or(EMPTY, new Or(EMPTY, gt1, gt2), gte3);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(GreaterThan.class, exp.getClass());
        GreaterThan gt = (GreaterThan) exp;
        assertEquals(ONE, gt.right());
    }
    
    public void testCombineBinaryComparisonsDisjunctionUpperBound() {
        FieldAttribute fa = getFieldAttribute();
        LessThan lt1 = lessThanOf(fa, ONE);