Last modified: 2014-06-30 13:38:25 UTC
That's to prevent breaking the whole system when there is no suitable DV-Handler.
I suggest to not have a null-handler, but instead catch the exception caused by not having a handler for some data value type. The exception should be logged using wfLogWarning (which causes tests to fail and gores into the error log in production).
Fix for this was merged a week ago https://github.com/wmde/WikibaseQueryEngine/pull/26