iOS - How to check null value? I will check but not working in iphone

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{

    if(receivedData)
    {
        id jsonObjects = [NSJSONSerialization JSONObjectWithData:receivedData options:NSJSONReadingMutableContainers error:nil];        
        
        id strBudname =  [NSString stringWithFormat:@"%@",[jsonObjects objectForKey:@"bmm_front"]];
       
        if (strBudname != [NSNull null]){
  
            img_url.text = strBudname;
        
        }else{
         
           img_url.text = @"";
           
        }

   }
    
}