fix: allow null argument for gender mapping

src/Twig/AppRuntime.php

@@ -95,8 +95,12 @@ class AppRuntime implements RuntimeExtensionInterface
         };
     }
 
-    public function mapGender(string $gender): string
+    public function mapGender(?string $gender): string
     {
+        if (null === $gender) {
+            return '';
+        }
+
         $gender = strtoupper($gender);
 
         return match ($gender) {