fix(contracts): return ok when amount is 0 to distribute

[DylanVerstraete]

When amount is 0 to distribute, return Ok so the calling function can continue it's work.

[renauter]

Ok, the idea here is, ideally, to make the user pay for amount_locked and if he has not enough funds to drain its account, correct?
And then reset its amount_locked to 0

Let s imagine an extreme case where usuable_balance = 1 and amount_locked = 1000
So the user will effectively pay 1 and the remaining 999 due will vanish
Is that what we want?

[DylanVerstraete]

yes, but this is only a viable flow in the case that a contract is in grace period or a contract got canceled. We cannot take more funds out of the users wallet then whatever he has left.

In the normal billing case, the user will always pay whatever he's due.

[renauter]

If I follow the logic above, if amount == 0 it is because user usable_balance == 0
Wouldn't be better if we handle this case at beginning of bill_contract()
Billing a user with no funds doesn't make sense...
Or maybe because bill_contract() does a lot more than billing the user 🥲

[DylanVerstraete]

bill_contract has to many flows to just return at the start of the function.

[renauter]

Other point
Why do we handle this case inside _distribute_cultivation_rewards()
Could be interesting to be more explicit upward like this?

        let billable_amount = twin_balance.min(contract_lock.amount_locked);

        if billable_amount != BalanceOf::<T>::saturated_from(0 as u128) {
            // Distribute cultivation rewards
            match Self::_distribute_cultivation_rewards(
                &contract,
                &pricing_policy,
                billable_amount,
            ) { ... }    
        }

[DylanVerstraete]

because we need to wrap an entire code block in an If-statement, I prefer the cleaner early return in the function.

[renauter]

LGTM
Just added some suggestions / ideas for billing redesign