Update zaphpa.lib.php

ADDED to Zaphpa_Callback_Util->getCallback a test for $callback === null along with a $file to allow for routing to static html pages without throwing the invalidcallback exception.

the add route then is

$router->addRoute(array(
  'path' => '/myfile',
  'get'  => null,
  'file' => 'myfile.html'
));