title: Kotlin Extension Functions - Override existing method
body: <p>is it possible to do something like:</p>
  
  <pre><code>/**
   * Converts all of the characters in the string to upper case.
   *
   * @param str the string to be converted to uppercase
   * @return the string converted to uppercase or empty string if the input was null
   */
  fun String?.toUpperCase(): String = this?.toUpperCase() ?: ""
  </code></pre>
  
  <ul>
  <li>What would this do?  It would make <code>toUpperCase</code> null safe.</li>
  <li>What problem am I having? the return value, <code>this?.toUpperCase()</code>,
  refers to the extension function</li>
  </ul>
  
  <p><strong>Is the only option to rename my extension function or is there a way to refer to the "super" function from within it?</strong></p>