Fix reachable unwrap on non-channel_type manual channel acceptance

Fix reachable unwrap on non-channel_type manual channel acceptance

If we receive an `OpenChannel` message without a `channel_type`
with `manually_accept_inbound_channels` set, we will `unwrap()`
`None`.

This is uncommon these days as most nodes support `channel_type`,
but sadly is rather trivial for a peer to hit for those with manual
channel acceptance enabled.

Reported in and fixes #2804. Luckily, the updated
`full_stack_target` has no issue reaching this issue quickly.