Fix $callback type for add_submenu_page (#161)

The default type for $callback is ''. Since there are parameters after $callback, we must support the default as well.

functionMap.php

@@ -25,6 +25,7 @@
  */
 return [
     'addslashes_gpc' => ['T', '@phpstan-template' => 'T', 'gpc' => 'T'],
+    'add_submenu_page' => [null, 'callback' => "''|callable"],
     'have_posts' => [null, '@phpstan-impure' => ''],
     'rawurlencode_deep' => ['T', '@phpstan-template' => 'T', 'value' => 'T'],
     'sanitize_category' => ['T', '@phpstan-template' => 'T of array|object', 'category' => 'T'],

wordpress-stubs.php

@@ -84252,6 +84252,7 @@ function add_menu_page($page_title, $menu_title, $capability, $menu_slug, $callb
      * @param callable  $callback    Optional. The function to be called to output the content for this page.
      * @param int|float $position    Optional. The position in the menu order this item should appear.
      * @return string|false The resulting page's hook_suffix, or false if the user does not have the capability required.
+     * @phpstan-param ''|callable $callback
      */
     function add_submenu_page($parent_slug, $page_title, $menu_title, $capability, $menu_slug, $callback = '', $position = \null)
     {