### 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/main/java/org/elasticsearch/xpack/ql/optimizer/OptimizerRules.java
main.source(),
                                    main.value(),
                                    lower ? main.lower() : other.lower(),
                                    lower ? main.includeLower() : other.includeLower(),
                                    upper ? main.upper() : other.upper(),
                                    upper ? main.includeUpper() : other.includeUpper(),
                                    main.zoneId()
                                )
                            );
                        }
                        
                        return compared;
                    }
                    
                    else {
                        
                        if (lower && upper) {
                            ranges.set(
                                i,
                                new Range(
                                    main.source(),
                                    main.value(),
                                    main.lower(),
                                    main.includeLower(),
                                    main.upper(),
                                    main.includeUpper(),
                                    main.zoneId()
                                )
                            );
                            return true;