Add a default value for the fiels history.data.

models/db1_history.py

@@ -16,7 +16,7 @@ db.define_table("history",
     Field("start_date", "date", default=today, notnull=True),
     Field("end_date", "date"),
     Field("note", "text"),
-    Field("data", "json"),
+    Field("data", "json", default=dict()),
     migrate="history.table")
 
 # NOTE: we have to remove the json validator to work !!