Fix KeyError: 'Result' when submitting AR analyses

http://sentry.bikalabs.com/bikalims/bikalims/group/322/

Don't know why, but sometimes (presumably related with 'Attachment
Outstanding' issue), when validating analyses from an AR, the submit
request is send without 'Result' GET value and  the system throws the
following exception:

Traceback (innermost last):
  Module ZPublisher.Publish, line 70, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.health.browser.analysisrequest.workflow, line 34, in __call__
KeyError: 'Result'

Added protective code against that