Proto,SCORU: Fix missing commit for empty inboxes trees

Context

To contribute to an inclusion proof, an Irmin tree must have been committed. The very first tree of messages on which an empty inbox is based was not committed.

Manually testing the MR

Before the MR (e.g., on master):

while dune exec src/proto_alpha/lib_protocol/test/unit/main.exe \
                -- test "^\[Unit\] sc rollup inbox$"; do; done

will ultimately fail.

After the MR, this does not look to be true anymore. (That's semidecidable, though.)

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