fix: Empty App Links

The return value of the function caused a warning
in header.inc.php:42 if no additional links were set.

header.inc.php is the only caller of this function,
thus it is safe to return an empty array here.

data/web/inc/functions.customize.inc.php

@@ -293,7 +293,7 @@ function customize($_action, $_item, $_data = null) {
           }
 
           if (empty($app_links)){
-            return false;
+            return [];
           }
 
           // convert from old style