Conditional in the preview API?

I am working currently on a project for a corporate client.
We are facing an issue with a preview and unfortunately, I couldn’t find information in the Preview API documentation.

What I would like to achieve is to generate a preview based on slug and category.
Unfortunately - the way we have all setup is that the category field should be empty as default.

The current preview looks like that:{entry.fields.pageType}/{entry.fields.slug}.

This works well with pages including category, but
with the empty field it returns NOT_FOUND in the URL. Is there any conditional possibility for that
case scenario to return the value only if present? If not: is this on your roadmap?
Thx in advance