Remove expect() in token transfer's acknowledgement conversion

[plafer]

I'm also not a fan of needing that comment. Note also that I added a test to check that this conversion doesn't return a non-empty vector.

The fix would look like:

impl From<TokenTransferAcknowledgement> for NonEmptyVec<u8> {
    // ...
}

impl From<TokenTransferAcknowledgement> for Acknowledgement {
    fn from(ack: TokenTransferAcknowledgement) -> Self {
        let v: NonEmptyVec<u8> = ack.into();

        // this can be an `into()` because the only possible error case
        // (empty vector) is not possible with `NonEmptyVec<u8>`
        v.into()
    }
}

Originally posted by @plafer in #377 (comment)