Fix integer nullable cast in protocol serializer

Since the class ExecuteBatchedStatement accepts nullable executorId it looks like it should cast to nullable int.

It occours using Isolate from multiple Flutter engines.

drift/lib/src/remote/protocol.dart

@@ -191,7 +191,7 @@ class DriftProtocol {
               list[0] as int, list.skip(1).toList()));
         }
 
-        final executorId = fullMessage.last as int;
+        final executorId = fullMessage.last as int?;
         return ExecuteBatchedStatement(
             BatchedStatements(sql, args), executorId);
       case _tag_RunTransactionAction: