fix(next): missing custom upload

[rilrom]

What?

Currently it is not possible to override the upload component.

Why?

The ability to override the upload component is missing from renderDocumentSlots.

How?

Adding a check to include the custom upload component if it is available.

I'm unsure if this is the correct approach to fix it but I have been running a patched version of payload with this change for a little while now with no issues.

This issue is holding me back from releasing a payload plugin.

Fixes #9591