### 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
"salary",
                "x"
            )
        );
    }
    private LocalRelation asEmptyRelation(Object o) {
        var empty = as(o, LocalRelation.class);
        assertThat(empty.supplier(), is(LocalSupplier.EMPTY));
        return empty;
    }
    private LogicalPlan plan(String query) {
        var analyzed = analyzer.analyze(parser.createStatement(query));
        
        var optimized = logicalOptimizer.optimize(analyzed);
        
        return optimized;
    }
    private LogicalPlan localPlan(LogicalPlan plan, SearchStats searchStats) {
        var localContext = new LocalLogicalOptimizerContext(EsqlTestUtils.TEST_CFG, searchStats);
        
        var localPlan = new LocalLogicalPlanOptimizer(localContext).localOptimize(plan);
        
        return localPlan;
    }
    @Override
    protected List<String> filteredWarnings() {
        return withDefaultLimitWarning(super.filteredWarnings());
    }
}