fix: allow optional values in query params

[wesbillman]

Fixes #1686

Here's the test I used:

type GetRequest struct {
	UserID string             `json:"userId"`
	PostID string             `json:"postId"`
	Tag    ftl.Option[string] `json:"tag"`
}

type GetResponse struct {
}

// Example:       curl -i http://localhost:8891/http/users/123/posts?postId=456&tag=foo
//
//ftl:ingress http GET /http/users/{userId}/posts
func Get(ctx context.Context, req builtin.HttpRequest[GetRequest]) (builtin.HttpResponse[GetResponse, string], error) {
	tag := req.Body.Tag.Default("No value")
	ftl.LoggerFromContext(ctx).Infof("Received request with tag: %s", tag)
	return builtin.HttpResponse[GetResponse, string]{
		Headers: map[string][]string{"Get": {"Header from FTL"}},
		Body:    ftl.Some(GetResponse{}),
	}, nil
}

curl -i http://localhost:8891/http/users/123/posts\?postId\=456\&tag\=foo

info:echo: Received request with tag: foo