Q-AJN-7: zero ERC20 transfer on claim reward

[grandizzy]

Description of change

High level

• Do not ERC20.transfer if rewards claimed are 0
  • check rewardClaimed_ is not 0 in claimDelegateReward() prior to transferring to the recipient

Description of bug or vulnerability and solution

• In the case where the caller doesn't have any rewards to claim, gas will be wasted on transferring a zero value.
  • if claimed rewards are 0 then do not attempt to transfer
  • _getDelegateReward better readability: minor change to calculate rewards if (votingPowerAllocatedByDelegatee != 0) instead if (votingPowerAllocatedByDelegatee == 0) return 0;
  • expose unit test

Contract size

Pre Change

| Contract              | Size (kB) | Margin (kB) |
|-----------------------|-----------|-------------|
| GrantFund             | 19.422    | 5.154       |

Post Change

| Contract              | Size (kB) | Margin (kB) |
|-----------------------|-----------|-------------|
| GrantFund             | 19.419    | 5.157       |

Gas usage

Pre Change

| Function Name                               | min             | avg    | median | max    | # calls |
| claimDelegateReward                         | 1407            | 57897  | 58540  | 65340  | 341     |

Post Change

| Function Name                               | min             | avg    | median | max    | # calls |
| claimDelegateReward                         | 1407            | 38175  | 36570  | 65359  | 11      |

[EdNoepel]

Thanks for adding the Transfer check.

[prateek105]

LGTM!

[MikeHathaway]

LGTM. Thanks for the fix