title: Assigning empty strings - throws exception
body: <p>Hi there
  I ran into this problem, where when i assign an empty string to a variable xcode throws an exception <code>[NSPlaceholderString initWithString:]: nil argument</code></p>
  
  <p>My scenario is</p>
  
  <p>&nbsp;1. Make a connection to server that gives xml.</p>
  
  <p>&nbsp;2. Am breaking up the xml and adding to the necessary variables as below  </p>
  
  <pre><code>NSString *city_xml=[[NSString alloc] init];
  NSString *state_xml=[[NSString alloc] init];
  NSString *country_xml=[[NSString alloc] init];
  
  city_xml=[TBXML valueOfAttributeNamed:@"name" forElement:city];
  state_xml=[TBXML valueOfAttributeNamed:@"state_name" forElement:city];
  country_xml=[TBXML valueOfAttributeNamed:@"country_name" forElement:city];
  NSLog(@"%@ %@ %@",city_xml,state_xml,country_xml);
  
  //release the strings after using them
  </code></pre>
  
  <p>When the xml returns an empty string, the said exception is raised.</p>
  
  <p>What is the commonly used way of handling this exception?</p>
  
  <p>I am using the following to workaround the problem...</p>
  
  <pre><code>if ([TBXML valueOfAttributeNamed:@"name" forElement:city]!=nil) {
      city_xml=[TBXML valueOfAttributeNamed:@"name" forElement:city];
  } else {
      city_xml=@"";
  }
  
  if ([TBXML valueOfAttributeNamed:@"state_name" forElement:city]!=nil) {
      state_xml=[TBXML valueOfAttributeNamed:@"state_name" forElement:city];
  } else {
      state_xml=@"";
  }
  
  if ([TBXML valueOfAttributeNamed:@"country_name" forElement:city]!=nil) {
      country_xml=[TBXML valueOfAttributeNamed:@"country_name" forElement:city];
  } else {
      country_xml=@"";
  }
  </code></pre>
  
  <p>But is there a proper way to handle this?</p>
  
  <p>Thanks</p>