Investigate: Connection's RawCounterparty.connection_id can be "" in the spec, but not our impl

[plafer]

This issues serves as a reminder to investigate the following.

The spec actually allows ConnectionEnd.counterpartyConnectionIdentifier to be "" (even though this is an illegal value for Identifier) - see issue. Our implementation disallows empty strings when converting from RawCounterparty -> Counterparty. This probably fine and due to the interplay between ibc-go's proto files using (gogoproto.nullable) = false, and how prost's code generation. We also successfully opened connections with basecoin-rs & hermes, which tells me it's right.

However, I want to convince myself that this code is correct, and document the reasoning somewhere.

[plafer]

Coming back to this issue, I see that I simply misread this expression. We indeed correctly don't disallow empty strings.