SCORU: Fix invalid empty inbox in rollup node

Context

The rollup node maintains its own inbox because this inbox contains all the history required to produce inclusion proofs for refutation games.

It is critical that the inbox descriptor of the rollup node stays consistent with the one computed by the layer 1 or else the refutation game will be lost by the rollup node.

This invariant is unfortunately invalid: adding this check in the rollup node makes the integration tests fail, witnessing a bug in the rollup node.

Fixes #3465 (closed) #3470 (closed).

Manually testing the MR

Before the commit fixing the bug, the following should fail:

dune exec tezt/tests/main.exe -- sc_rollup inbox

After the bugfix commit, the CI must be green again.

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