Add page callback to default menu item

There is a new patch for Drupal core that is scheduled for release in 7.50: https://www.drupal.org/node/1081266?page=1#comment-10648642 This patch is used to avoid re-scanning the module directory when a filename or a module is missing. It also presents an error message on all pages if a filename or module is deemed to be missing:

+    if ($error_type == 'missing') {
+      trigger_error(format_string('The following @type is missing from the file system: %name. In order to fix this, put the @type back in its original location. For more information, see <a href="@documentation">the documentation page</a>.', array('@type' => $type, '%name' => $name, '@documentation' => 'https://www.drupal.org/node/2487215')), E_USER_WARNING);
+    }

Since we applied the core patch, we have started seeing error messages about a missing module. However, Drupal does not tell us what module is causing the problem:

User warning: The following module is missing from the file system: . In order to fix this, put the module back in its original location. For more information, see the documentation page. in trigger_error() (line 1079 of /includes/bootstrap.inc)

We did see a similar issue with Elysia Cron (https://www.drupal.org/node/2407129), and used this as a guideline to search for possible menu items that might be causing a problem.

Sure enough, we found that the issue is the default menu item created by Views UI Basic. It seems as though since it does not have a page callback, Drupal core considers the module to be missing. By adding a callback and arguments, the problem is fixed.

I'll attach a patch in a few minutes for review.