### 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
And and = new And(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(and);
        assertEquals(r1, exp);
    }
    
    public void testCombineBinaryComparisonsConjunctionOfNonOverlappingBoundaries() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, ONE, false, TWO, false);
        And and = new And(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(and);
        assertEquals(Range.class, exp.getClass());
        Range r = (Range) exp;
        assertEquals(TWO, r.lower());
        assertFalse(r.includeLower());
        assertEquals(TWO, r.upper());
        assertFalse(r.includeUpper());
        assertEquals(Boolean.FALSE, r.fold());
    }
    
    public void testCombineBinaryComparisonsConjunctionOfUpperEqualsOverlappingBoundaries() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, TWO, false, THREE, true);
        And and = new And(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(and);
        assertEquals(r1, exp);