Page being a child of itself

This happened to us in our live environment. A page somehow became a child of itself.

## To Reproduce

In reality **this happened through use of the UI only**, we just don't know how. However, I know how to manipulate the database to replicate the issue.

1. Choose a page to manipulate. I'd recommend using one with type `default-page`.
2. Change the page's `path` property. A normal path might look like this: `"{home-page-id}/{page-id}"`,
change this to `"{home-page-id}/{page-id}/{page-id}"`

## Expected behavior
I don't really know, I guess the expected behaviour would be for this not to happen in the first place.

## Describe the bug
After following the steps above, the page is now a child of itself.
This comes with some side effects:
- The page does not show up in the page tree.
- The page can not be saved, as it has a draft parent (itself), which has a draft parent (itself again), etc.

## Details

**Server Operating System:**
`Ubuntu 22.04.2 LTS (GNU/Linux 5.15.0-112-generic x86_64)` with `node:lts-alpine3.15` Docker image

**Screenshots:**
The message that pops up when trying to save this page:

Start by reproducing the database path change described in the issue and inspect the page hierarchy and save flow involved when a page becomes its own parent. Done should include preventing this invalid relationship through the UI or data handling and restoring a page that is already self-parented, with the page tree and save operation working normally.

Written by the indexing model from the issue text.