Any argument after a missing one is silently ignored (D7)

How to reproduce using panels

How to reproduce:
* Add a view with a panels pane display and 2 arguments (aka contextual filters), both populated from an optional context
* Configure it with Arg1=No-context, Arg2=Some-context

Expected: View is filtered with argument2

Experienced: View is not filtered, argument2 silently ignored

How to reproduce with code

I have a view with 2 contextual filters, changed date and node type. Both filters are set up so that all values are returned if the argument is NULL. As such, I expect to be able to both arguments independently, giving 4 possible combinations:

This code demonstrates it:

  $view = views_get_view('test');

  $args = array('2011', 'paper');    // works fine, all 'paper' nodes created in 2011
  $args = array('2011', NULL);       // works as expected, all nodes created in 2011
  $args = array(NULL, 'paper');      // returns all nodes, NOT only nodes of type paper
  $args = array(NULL, NULL);         // returns all nodes as expected

  return $view->preview('default', $args);

The _build_arguments function in view.inc seems to stop whenever it encounters an argument that is not obtained from the url and has no value. Subsequent arguments are therefore ignored.