Translations dashboard errors on missing languages for a page

[David Callé]

Internal Server Error: /translations-dashboard/
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/cache.py", line 52, in _wrapped_view_func
    response = view_func(request, *args, **kwargs)
  File "/srv/devportal-app-0/code/142/translations_dashboard/views.py", line 37, in translations_dashboard
    {'cms_pages': get_pages()},
  File "/srv/devportal-app-0/code/142/translations_dashboard/views.py", line 29, in get_pages
    dashboard_data, key=lambda k: k.get('en', None))))
TypeError: can't compare datetime.datetime to NoneType

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Pages missing the 'en' language (which are a handful of very old pages that predates the public launch of the site) need to return a valid time instead of None.