NSDictionary <null> 값 비교 처리

# 방법 1

if([[dictionary objectForKey:@"Field"] isKindOfClass:[NSNull class]]){
}

# 방법 2

if ([dictionary objectForKey:@"Field"] == (NSString *)[NSNull null]){
}

# 방법 3

if ([dictionary objectForKey:@"Field"] == (id)[NSNull null]) {
}