Do not create a `proposal.pay` entry when the amount in 0

[jeisses]

If a proposal does not have a reward, it should not create a pay entry in the blockchain.
This is wasted RAM and also introduced a bug that the proposal can note be executed because the amount is not positive.
This bug has been fixed on the smart contract side; however for clarity and efficiency we should still not add 0-pay reward entries

Steps

• On proposal create and proposal edit, check if the reward is 0. In this case, submit an empty array in the proposal.pay field.