Views contextual filters programmatically alter add to or
If you have a way of knowing both filter values (like NULL and 12), then I would use hook_views_pre_build and unset the NULL field.

function test_views_pre_build(&amp;$view) {
  // Do some checks for view name and current_display
  unset($view-&gt;display_handler-&gt;handlers['argument']['null_field']);
}