Fix error when submitting an Analysis Request without any analysis being checked

http://sentry.bikalabs.com/bikalims/bikalims/group/643/
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.analysisrequest.workflow, line 51, in __call__
  Module bika.lims.browser.analysisrequest.workflow, line 290, in workflow_action_submit
KeyError: 'Result'