Fix 'NoneType' object has no attribute 'asdatetime' in Instrument.getValidCertifications()

http://sentry.bikalabs.com/bikalims/bikalims/group/424/
1393919993.720.927781276061 http://demo.bikalabs.com/get_instruments_outofdate
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.instrument, line 599, in __call__
  Module bika.lims.content.instrument, line 253, in isOutOfDate
  Module bika.lims.content.instrument, line 269, in getLatestValidCertification
AttributeError: 'NoneType' object has no attribute 'asdatetime'