Proto: fix error raised when no commitment can be cemented

Context

Closes #4530 (closed)

The current error is invalid as it can says "disputed" if the list of commitments is empty. Unfortunately (but not that much), it can not be tested, as such a situation would make the application fail earlier.

Does not require a backport.

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