It was possible to define empty structs without any fields and right after, trigger the EIP712 sign UI flow for blank domain & message hashes.
Added checks if there is actually anything relevant to sign.
* Possible security fix.
It is possible to send a new structure definition after sending a structure implementation, which makes the app treat unrestricted data as if it was a well defined structure.
This commit tries to fix that behaviour. Once a structure implementation is sent, we consider all structures to be defined and we do not allow new definitions.
* Fix previous commit
