Handle empty inputs in Redis and database operations, improve safety seal validation, and enhance related tests

- Return empty arrays for empty inputs in Redis `mget`, `db_object_t`, and `users_o` operations.
- Refactor safety seal validation logic to handle numeric strings and improve clarity.
- Add unit and API tests to verify handling of empty inputs and numeric safety seal strings.

services/nginx/app/objects/users_o.php

@@ -1534,6 +1534,10 @@ class users_o extends db
     {
         global $db;
         $customer_numbers = array_map('intval', $customer_numbers);
+        if (empty($customer_numbers)) {
+            return [];
+        }
+
         // Look in the cache first
         $customer_numbers_to_fetch = [];
         $customer_names_cached = self::getCachedForMultipleObjects('economic_customer_name', $customer_numbers);