### 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
PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new And(EMPTY, eq1, eq2));
        assertEquals(FALSE, exp);
    }
    
    public void testDualNullEqualsConjunction() {
        FieldAttribute fa = getFieldAttribute();
        NullEquals eq1 = nullEqualsOf(fa, ONE);
        NullEquals eq2 = nullEqualsOf(fa, TWO);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new And(EMPTY, eq1, eq2));
        assertEquals(FALSE, exp);
    }
    
    public void testEliminateRangeByEqualsOutsideInterval() {
        FieldAttribute fa = getFieldAttribute();
        Equals eq1 = equalsOf(fa, L(10));
        Range r = rangeOf(fa, ONE, false, L(10), false);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new And(EMPTY, eq1, r));
        assertEquals(FALSE, exp);
    }
    
    public void testEliminateRangeByNullEqualsOutsideInterval() {
        FieldAttribute fa = getFieldAttribute();
        NullEquals eq1 = nullEqualsOf(fa, L(10));
        Range r = rangeOf(fa, ONE, false, L(10), false);
        PropagateEquals rule = new PropagateEquals();
        Expression exp = rule.rule(new And(EMPTY, eq1, r));
        assertEquals(FALSE, exp);