Fix AttributeError when DateOfBirth field is hidden in add sample

[xispa]

Description of the issue/feature this PR addresses

This Pull Request fixes an error that arises on sample creation when the field DateOfBirth is disabled from the sample add form via "Manage Form Fields".

Current behavior before PR

Traceback (innermost last):
  Module ZServer.ZPublisher.Publish, line 144, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZServer.ZPublisher.Publish, line 44, in call_object
  Module bika.lims.decorators, line 51, in decorator
  Module bika.lims.browser.analysisrequest.add2, line 708, in __call__
  Module bika.lims.browser.analysisrequest.add2, line 1745, in ajax_submit
  Module senaite.patient.browser.widgets.agedob, line 64, in process_form
AttributeError: 'str' object has no attribute 'get'

Desired behavior after PR is merged

No traceback. The sample is created successfully

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.