Using NULL_SENDER tx to send vote tx, it's a new transaction format defined in EIP86.

1. If the signature of a transaction is (0, 0, 0) (ie. v = r = s = 0), then treat it as valid and set the sender address to 2**160 - 1
2. nonce = gasprice = value = 0

See: https://github.com/karlfloersch/pyethereum/blob/develop/ethereum/transactions.py#L82-L85

Related discussions:

1. Proposed initial abstraction changes for Metropolis EIPs#86
2. Abstraction of transaction origin and signature.md EIPs#208