DatabaseBob.py : method fails if model_id has the label '0'

Created by: tiagofrepereira2012

There is a flaw in the method probe_file_sets in the script bellow. https://github.com/bioidiap/bob.bio.base/blob/master/bob/bio/base/database/DatabaseBob.py#L328

Depending of the database the method will never return the correct probes if the model_id is actually 0.

This test should be replaced by a wiser test

if model_id is not None:

A patch is on the way.