Allow 0 as nonce for offline transaction

[markusschaden]

This pull request fixes the issue that it is not possible to create a offline signature from an address which has no transactions so far (nonce 0)

If you try to sign a tx with nonce 0, it results in the following error and the signing process starts all over again, enter transaction id

[denisgranha]

Thank your for taking the time of doing this

[vogelito]

yay! merged!