fix: convertWithSign.php: add class existence check

Added a condition to check if the class 'QUI\\ERP\\Money\\Price' exists before executing the rest
of the code in the convertWithSign.php file in the ajax directory. This fix prevents potential
errors and improves code stability.

Related: #9

ajax/convertWithSign.php

@@ -18,6 +18,10 @@ function ($data) {
         $data = json_decode($data, true);
         $result = [];
 
+        if (!class_exists('QUI\ERP\Money\Price')) {
+            return $result;
+        }
+
         foreach ($data as $entry) {
             $amount = $entry['amount'];
             $currencyFrom = $entry['from'];