[Bug]: TxBuilder does not allow to SetSignatures with an empty Public Key

[cmwaters]

Is there an existing issue for this?

• I have searched the existing issues

What happened?

As far as I can understand from the code, it is not compulsory to include the public key in the AuthInfo of the transaction. It is predominantly needed for the first transaction used by an account and is afterwards saved to state so following transaction don't need it.

I would have expected to be able to run SetSignatures with the PubKey field left empty but I get the following error:

panic: error setting draft signatures: Expecting non nil value to create a new Any: failed packing protobuf message to Any [celestiaorg/cosmos-sdk@v1.20.0-sdk-v0.46.16/codec/types/any.go:61] [recovered]

Cosmos SDK Version

v0.46 and later

How to reproduce?

Have a test call SetSignatures with the PubKey field empty.

[cmwaters]

I'm working on a fix here celestiaorg#371 on my fork of the SDK that I'd be happy to upstream

[alexanderbez]

@cmwaters the proposal makes sense to me and it seems erroring should not happen. Would be happy to review and accept a PR.