Add zero claim check

packages/contracts-bedrock/.gas-snapshot

@@ -38,7 +38,7 @@ FaultDisputeGame_Test:test_gameStart_succeeds() (gas: 10315)
 FaultDisputeGame_Test:test_gameType_succeeds() (gas: 8237)
 FaultDisputeGame_Test:test_initialRootClaimData_succeeds() (gas: 19522)
 FaultDisputeGame_Test:test_rootClaim_succeeds() (gas: 8225)
-FaultDisputeGame_Test:test_simpleAttack_succeeds() (gas: 129016)
+FaultDisputeGame_Test:test_simpleAttack_succeeds() (gas: 129033)
 FaultDisputeGame_Test:test_version_succeeds() (gas: 9736)
 FeeVault_Test:test_constructor_succeeds() (gas: 18185)
 GasBenchMark_L1CrossDomainMessenger:test_sendMessage_benchmark_0() (gas: 352135)

packages/contracts-bedrock/contracts/dispute/FaultDisputeGame.sol

@@ -132,6 +132,11 @@ contract FaultDisputeGame is IFaultDisputeGame, Clone {
             revert GameNotInProgress();
         }
 
+        // The zero hash is not a valid claim.
+        if (Claim.unwrap(pivot) == bytes32(0)) {
+            revert InvalidClaim();
+        }
+
         // The only move that can be made against a root claim is an attack. This is because the
         // root claim commits to the entire state; Therefore, the only valid defense is to do
         // nothing if it is agreed with.

packages/contracts-bedrock/contracts/libraries/DisputeErrors.sol

@@ -39,6 +39,11 @@ error CannotDefendRootClaim();
  */
 error ClaimAlreadyExists();
 
+/**
+ * @notice Thrown when a given claim is invalid (0).
+ */
+error InvalidClaim();
+
 /**
  * @notice Thrown when an action that requires the game to be `IN_PROGRESS` is invoked when
  *         the game is not in progress.