fix: encode fail when the sequence is blank
fix: encode fail when the sequence is blank

vaporry/pyethereum,ddworken/pyethereum,ebuchman/daoist_protocol,ethers/pyethereum,pipermerriam/pyethereum,karlfloersch/pyethereum,ethereum/pyethereum,pipermerriam/pyethereum,holiman/pyethereum,jnnk/pyethereum,ebuchman/daoist_protocol,ethermarket/pyethereum,shahankhatch/pyethereum,ethereum/pyethereum,shahankhatch/pyethereum,ckeenan/pyethereum,inzem77/pyethereum,harlantwood/pyethereum,karlfloersch/pyethereum