scoru: fix inbox flaky test

Context

There is a pbt that was flaky. The problem was that the generator could creates a origination level for an inbox of Raw_level_repr.root which is impossible and enforce by a assert in Sc_rollup_inbox_repr.empty.

This simply fixes that.

Manually testing the MR

Checklist

- [ ] Document the interface of any function added or modified (see the coding guidelines) - [ ] Document any change to the user interface, including configuration parameters (see node configuration)

• Provide automatic testing (see the testing guide). - [ ] For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR