Error when predicting using an older decision tree

[Rahul Iyer]

An error was found when predicting using MADlib v1.12, with the decision
tree trained using an older version. The details can be found in MADLIB-1161
<https://issues.apache.org/jira/browse/MADLIB-1161>.

The workaround for the error is to add an additional column called
'null_proxy' with a NULL value.

ALTER TABLE <summary_table> ADD COLUMN null_proxy TEXT DEFAULT NULL;