fix: frozen tokens for null address

contracts/src/kleros-v1/KlerosV1Governor.sol

@@ -139,7 +139,7 @@ contract KlerosV1Governor is IArbitrable, ITokenController {
  for (uint256 voteID = 0; voteID < votesLengths[round]; voteID++) {
  (address account, , , ) = klerosLiquid.getVote(_disputeID, round, voteID);
  require(account != address(0x0) || isDrawingForbidden, "Juror not drawn yet.");
- frozenTokens[account] += tokensAtStakePerJuror[round];
+ if (account != address(0x0)) frozenTokens[account] += tokensAtStakePerJuror[round];
  }
  isDisputeNotified[_disputeID][round] = true;
  }