Fix generation inputs logging pre-transaction execution

[Nashtare]

Yet another tiny bug fix post #56.
CPU simulation displays default values in Debug mode, because we initialize the interpreter with empty inputs, before setting them during preinitialization.

DEBUG evm_arithmetization::generation::state: Input signed_txn: None    
DEBUG evm_arithmetization::generation::state: Input state_trie: HashedPartialTrie { node: Empty, hash: RwLock { data: None } }    
DEBUG evm_arithmetization::generation::state: Input transactions_trie: HashedPartialTrie { node: Empty, hash: RwLock { data: None } }    
DEBUG evm_arithmetization::generation::state: Input receipts_trie: HashedPartialTrie { node: Empty, hash: RwLock { data: None } }    
DEBUG evm_arithmetization::generation::state: Input storage_tries: []    
DEBUG evm_arithmetization::generation::state: Input contract_code: {}    

This PR fixes it.

[wborgeaud]

LGTM