fix: currency service error

app/Services/CurrencyService.php

 
 class CurrencyService implements CurrencyHandler
 {
-    protected string $api_key;
+    protected string|null $api_key;
 
-    protected string $base_url;
+    protected string|null $base_url;
 
     protected Client $client;
 
-    public function __construct(string $api_key, string $base_url, Client $client)
+    public function __construct(string|null $api_key, string|null $base_url, Client $client)
     {
         $this->api_key = $api_key;
         $this->base_url = $base_url;