When doing Ethereum signTypedData, and the primaryType="EIP712Domain",
we completely ignore the "message" part and only sign the domain.
According to the community, this is technically allowed by the spec,
and may be used by ETH smart contracts to save on gas.
Test case generated by @MetaMask/eth-sig-util's library.
See: https://ethereum-magicians.org/t/eip-712-standards-clarification-primarytype-as-domaintype/3286
