### 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
Range r = (Range) exp;
        assertEquals(ONE, r.lower());
        assertFalse(r.includeLower());
        assertEquals(FOUR, r.upper());
        assertFalse(r.includeUpper());
    }
    
    public void testCombineBinaryComparisonsDisjunctionOfNonOverlappingBoundaries() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, ONE, false, TWO, false);
        Or or = new Or(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(or, exp);
    }
    
    public void testCombineBinaryComparisonsDisjunctionOfUpperEqualsOverlappingBoundaries() {
        FieldAttribute fa = getFieldAttribute();
        Range r1 = rangeOf(fa, TWO, false, THREE, false);
        Range r2 = rangeOf(fa, TWO, false, THREE, true);
        Or or = new Or(EMPTY, r1, r2);
        CombineBinaryComparisons rule = new CombineBinaryComparisons();
        Expression exp = rule.rule(or);
        assertEquals(r2, exp);
    }
    
    public void testCombineBinaryComparisonsOverlappingUpperBoundary() {
        FieldAttribute fa = getFieldAttribute();
        Range r2 = rangeOf(fa, TWO, false, THREE, false);