Minor fix for return type alignment

make sure string returned is set to string in config getter

psalm.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0"?>
 <psalm
-    errorLevel="3"
+    errorLevel="8"
     resolveFromConfigFile="true"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="https://getpsalm.org/schema/config"

src/Config/Config.php

@@ -85,7 +85,7 @@ class Config implements ConfigInterface
 			case 'AWS_GIFT_CARD_PARTNER_ID':
 			case 'AWS_GIFT_CARD_ENDPOINT':
 			case 'AWS_GIFT_CARD_CURRENCY':
-				$return = $_ENV[$key] ?? '';
+				$return = (string)($_ENV[$key] ?? '');
 				break;
 			default:
 				break;