title: Composer error when running specific commands
body: <p>I'm trying to run specific composer commands like update, self-update or diagnose and I keep getting the error below when: </p>
  
  <pre><code>  Argument 1 passed to Composer\Config::merge() must be of the type array, null given, called in phar:///usr/local/Cellar/composer/1.0.0-alpha8/libexec/composer.phar/src/Composer/Factory.php on line 92 and defined 
  </code></pre>
  
  <p>Other commands like validate, init or about are working. Also running validate doesn't show any errors in my composer.json file.</p>
  
  <p>Things tried to no avail:</p>
  
  <ul>
  <li>used PHP version shipped with OSX</li>
  <li>used homebrew PHP version</li>
  <li>downloaded and used composer.phar from getcomposer</li>
  <li>used homebrew composer</li>
  </ul>
  
  <p>Has anybody encountered the error and can give advice please?</p>
  
  <p>Cheers,</p>
  
  <p>Stephan</p>