NSInteger returning 0 if NSString is NULL
===

This could be the simplest one but I am not able to find the exact cause. Instead to explain in words following is my code:

    NSString *str = @"";
    NSInteger tempInteger = [str integerValue];
    
    if (tempInteger == 0)
    {
      NSLog (@"do something");
    }

If NSString returning NULL then NSInteger returning 0 and if NSString has some value then NSInteger returning same value. 

But when NSString is NULL then I want NSInteger should return NULL not 0 but I am not able to validation NSInteger as NULL. 

How can I get NULL value in NSInteger if NSString is NULL?