Bug in IbcQuery::ListChannels implementation when port is unset

[webmaster128]

According to the API specification, an unset port ID falls back to the contract's port ID:

If port_id is omitted, this list all channels bound to the contract's port.

(https://github.com/CosmWasm/cosmwasm/blob/v1.3.3/packages/std/src/query/ibc.rs#L16-L20), and

If PortID is unset, this list all channels bound to the contract's port.

(https://github.com/CosmWasm/wasmvm/blob/v1.3.0/types/queries.go#L165-L172). This specification never changed since the API's existence.

However, in https://github.com/CosmWasm/wasmd/pull/439/files#diff-47293308b181e6cdf9a982870e2de9941dfb405553ed239f7a192ec46963ec58R180 the implemetation changed to list channels for all ports in that case, instead of the contract's port. This is a bug in wasmd.

The motivation for changing the implementation instead of documenting the current behaviour is the following:

• The behaviour of falling back to the contract's port is consistent with IbcQuery::Channel
• Listing all channels of the chain is hardly something that makes sense to do in a smart contract
• It seems like the query is hardly used. After asking around only one user was found: astroport. That user can probably better use a different query.