### 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/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/LocalLogicalPlanOptimizerTests.java
var source = as(limit.child(), EsRelation.class);
    }
    /**
     * Expects
     * LocalRelation[[first_name{f}#4],EMPTY]
     */
    public void testMissingFieldInFilterNumericWithReference() {
        var plan = plan("""
              from test
            | eval x = emp_no
            | where x > 10
            | keep first_name
            """);
        var testStats = statsForMissingField("emp_no");
        var localPlan = localPlan(plan, testStats);
        var local = as(localPlan, LocalRelation.class);
        assertThat(Expressions.names(local.output()), contains("first_name"));
    }
    /**
     * Expects
     * LocalRelation[[first_name{f}#4],EMPTY]
     */
    public void testMissingFieldInFilterNumericWithReferenceToEval() {
        var plan = plan("""
              from test
            | eval x = emp_no + 1
            | where x > 10
            | keep first_name
            """);
        var testStats = statsForMissingField("emp_no");