Check if partner submitted balance proof before settling

[LefterisJP]

Problem Definition

Part of #2214

During settle at the moment we always expect the partner to have succesfully called updateTransfer. That is an assumption that may not be true.

Tasks

• Check the blockchain and see if there is no balance proof submitted by the partner of the channel
• If there is nothing submitted it means the partner did not submit his side of the channel so don't use the expected partner's balance proof and instead use empty proof