Fix optional return type

qbittorrent-web-api-gen/src/generate/group.rs

@@ -97,6 +97,12 @@ impl types::Type {
             },
         };
 
+        let owned_type = if self.is_optional() {
+            quote! { std::option::Option<#owned_type> }
+        } else {
+            owned_type
+        };
+
         if self.is_list() {
             quote! { std::vec::Vec<#owned_type> }
         } else {