Returns nil if equal to [NSNull null] else return self

NSNull+SelfOrNil.h

//
// Usage: `[item.self anyMethod]` is valid for `item` equals to `[NSNull null]`
//
@interface NSNull (SelfOrNil)

@end

NSNull+SelfOrNil.m

@implementation NSNull (SelfOrNil)

- (instancetype)self {
    return nil;
}

@end