Fix EIP-712 domain map containing unused field

[stevenleeg]

This fixes an issue I noticed with #17789 (cc @PaulRBerg) which resulted in a bad PrettyPrint of the Domain struct when attempting to debug message signing.

The original code makes the assumption that setting a key in a map to nil results in a noop, however in Go the key is still added to the map with its value set to nil, which yields a value showing up in TypedData.Format() even though it may not be specified by the user's TypedData struct.

This PR fixes this, moving domain.ChainId from the map's initializer down to a separate if statement which checks the existance of ChainId's value, similar to the rest of the fields, before adding it. I've also included a new test to demonstrate the issue

[holiman]

Hm. But is it really valid without a domain ? Isn't that required?

[kumavis]

@holiman not optional domain but optional domain.chainId

[stevenleeg]

Yup, domains aren't any less required, this just makes it so chainId doesn't show up for users who chose to leave out that specific field (just as it does for the other fields).

Also not super sure what's going on with the CI– it appears my test is passing but something else is causing it to fail elsewhere?