Update DocBlock for Zend_Console_Getopt::getOption()

Closes #599

library/Zend/Console/Getopt.php

@@ -550,7 +550,7 @@ class Zend_Console_Getopt
      * current application invocation.
      *
      * This function returns true, or the parameter value to the option, if any.
-     * If the option was not given, this function returns false.
+     * If the option was not given, this function returns null.
      *
      * @param  string $flag
      * @return mixed