If we process the data which contains the missing value("?"),
the tree cannot be created because DataConverter.convert inserts the null value
to the list of Instances.
Of cause we can fix this issue with prohibiting DataConverter.convert insert
the null value, but I notice that there is a potentiality that the rows
which have missing value("?") can be also used to make the tree.
We can use them for making all stems on the edge where we use the missing value.