fix for empty sender
fix for empty sender

vaporry/pyethereum,shahankhatch/pyethereum,karlfloersch/pyethereum,harlantwood/pyethereum,ethereum/pyethereum,pipermerriam/pyethereum,holiman/pyethereum,ethereum/pyethereum,ddworken/pyethereum,pipermerriam/pyethereum,ckeenan/pyethereum,inzem77/pyethereum,karlfloersch/pyethereum,shahankhatch/pyethereum