title: Objective-C how to check if a string is null
body: <p>SO
  I wish to check to see if the item in my array <code>[clientDataArray objectForKey:@"ClientCompany"]</code> is <code>nil</code>.</p>
  
  <pre><code>    temp = [clientDataArray objectForKey:@"ClientCompany"];
      if (temp != [NSNull null]) infofieldCompany.text = temp;
  </code></pre>
  
  <p>So far I have been able to achieve this through the above code, but it does give me the warnings</p>
  
  <ul>
  <li>warning: <code>NSArray</code> may not respond to <code>-objectForKey:</code></li>
  <li>warning: comparison of distinct
  Objective-C types <code>struct NSNull *</code>
  and <code>struct NSString *</code> lacks a cast</li>
  </ul>
  
  <p>My main interest is the second warning, but the first warning also interest me. 
  How should I adapt my above code?</p>