The simple way to make NSNull act as nil: to respond with `0` (==`nil`) to any message.

null.m

/*
 	clang -o null -framework Foundation null.m
	ericbroska
 */
#import <Foundation/Foundation.h>
#include <objc/objc-class.h>
#include <objc/runtime.h>

int main(int argc, char const *argv[])
{
	@autoreleasepool {
		/* voids */
		[[NSNull null] performSelector: @selector(magic)];
		[[NSNull null] start];
		[[NSNull null] addObserver: nil
		                forKeyPath: @"kNullWeirdKeyPath"
		                   options: 0
		                   context: [NSString stringWithString: @"something"]];

		NSLog(@"%@ : %lu", @"NSUInteger",   [[NSNull null] count]);
		NSLog(@"%@ : %d",  @"BOOL/Integer", [[NSNull null] isLessThan: @3.14 ]);
		NSLog(@"%@ : %c",  @"Char",         [[NSNull null] characterAtIndex: 51] ?: '?');
		NSLog(@"%@ : %@",  @"Object",       [[NSNull null] objectForKey: @"key"]);
	}
	
	return 0;
}


int _placeholder(id self, SEL selector, ...)
{
	return 0;
}

@implementation NSNull (TryToFailMe)
+ (BOOL)resolveInstanceMethod:(SEL)aSEL
{
	return class_addMethod([self class], aSEL, (IMP)_placeholder, "i@:?");
}
@end