This is fixes one regression from Hugo `0.42.1`:

* Fix type checking in Append [2159d77f](https://github.com/gohugoio/hugo/commit/2159d77f368eb1f78e51dd94133554f88052d85f) [@bep](https://github.com/bep) [#5303](https://github.com/gohugoio/hugo/issues/5303)