Ensure received emails with no chain are signed correctly

[abeverley]

Fixes #169. If OpenARC is used to verify an inbound email (with no existing chain), then if the email is modified and re-signed, then the chain is broken. This is because OpenARC carries forward the existing none verification status, rather than changing it to a pass as would be expected from this scenario functioning correctly.

This commit ensures that a message that was received with none which now has one chain has its cv changed to pass instead of remaining at none.