Check for template_include passing non-strings. Fixes #1134

I agree that the plugin should probably be using exit() and I submitted a bug report. But I don't often get responses on wordpress.org plugin support forums so I thought I'd try my luck here.

To generate the warning ("PHP Warning: include(): Filename cannot be empty in /var/vhosts/.../base.php on line 17"), add this to functions.php and try to load the homepage:

    add_filter('template_include', function ($template) {
      return null;
    });

Applying this patch makes that warning go away.

Thanks.