root level paths `/` are not split correctly

[LukeHagar]

Describe the bug

When running the redocly split command the openapi path at the API root: / is not properly moved to the paths folder, and the references in the document are not properly converted to file based references.

To Reproduce Steps to reproduce the behavior:

Minimum repro repo: https://github.com/LukeHagar/redocly-split-repro

1. Open the repo
2. Run make repro
3. See the file here: https://github.com/LukeHagar/redocly-split-repro/blob/main/openapi/paths.yaml

Expected behavior

I would expect this path to be handled like all the others

Redocly Version(s)

1.18.0

Node.js Version(s)

v20.15.0

Additional context

You rock! I love the bundle and split implementations