Set random in CeloBackend to ensure IsMerge

If this is not set, the PUSH0 instruction is not enabled, since both
IsMerge and IsShanghai have to be true to enable it. Not enabling it can
lead to reverts when calling core contracts from within op-geth (not via
RPC).

contracts/celo_backend.go

@@ -40,7 +40,11 @@ func (b *CeloBackend) CallContract(ctx context.Context, call ethereum.CallMsg, b
 	if blockNumber == nil {
 		blockNumber = common.Big0
 	}
-	blockCtx := vm.BlockContext{BlockNumber: blockNumber, Time: 0}
+	blockCtx := vm.BlockContext{
+		BlockNumber: blockNumber,
+		Time:        0,
+		Random:      &common.Hash{}, // Setting this is important since it is used to set IsMerge
+	}
 	txCtx := vm.TxContext{}
 	vmConfig := vm.Config{}