FIX failing queries on datetime fields set to 1970-01-01 00:00:00 via (int) 0

[hsegnitz]

The Problem was the not-type-safe null-check in the PropelDateTime util class. Passing an Integer 0 (that e.g. comes from the SplFileInfo::getMtime()) to the setter of the field, results in the 0 being interpreted as null, thus the update/insert query is failing.

FIxes #2010