encoding removes empty slices which makes returning empty arrays from ingress fail

[wesbillman]

If a project has an http ingress with a response spec that might return something like this

{
  "data": []
}

Where the API specifies it will return an empty array, we currently strip the "data" field from the json response because of our encoding.

type GoodResponse struct {
	Data []string `json:"data"`
}

//ftl:export
//ftl:ingress GET /good
func Good(ctx context.Context, req builtin.HttpRequest[GoodRequest]) (builtin.HttpResponse[GoodResponse, builtin.Empty], error) {
	resp := GoodResponse{Data: []string{}}
	return builtin.HttpResponse[GoodResponse, builtin.Empty]{
		Body:   ftl.Some(resp),
	}, nil
}

The code above will return {} where the user and API wants {"data":[]}.

This seems to be happening here https://github.com/TBD54566975/ftl/blob/main/go-runtime/encoding/encoding.go#L119-L125.

Should we add some logic to honor omitempty for fields like this? Or remove the stripping of empty fields here? Or is there a better option for users?

Currently, the work around is to use a response type of []byte and handle the json encoding manually in the user code.

cc: @mistermoe