### 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
for (int i = 0; i < ranges.size(); i++) { 
                    Range range = ranges.get(i);
                    if (eq.left().semanticEquals(range.value())) {
                        Integer lowerComp = range.lower().foldable() ? BinaryComparison.compare(eqValue, range.lower().fold()) : null;
                        Integer upperComp = range.upper().foldable() ? BinaryComparison.compare(eqValue, range.upper().fold()) : null;
                        if (lowerComp != null && lowerComp == 0) {
                            if (range.includeLower() == false) { 
                                ranges.set(
                                    i,
                                    new Range(
                                        range.source(),
                                        range.value(),
                                        range.lower(),
                                        true,
                                        range.upper(),
                                        range.includeUpper(),
                                        range.zoneId()
                                    )
                                );
                            } 
                            removeEquals = true; 
                            break;
                        } else if (upperComp != null && upperComp == 0) {
                            if (range.includeUpper() == false) { 
                                ranges.set(
                                    i,
                                    new Range(
                                        range.source(),
                                        range.value(),