[jb] Opening a job might throw a 500 error if its tasks are None

File "/usr/lib/hue/build/env/lib/python2.6/site-packages/Django-1.6.10-py2.6.egg/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/hue/build/env/lib/python2.6/site-packages/Django-1.6.10-py2.6.egg/django/db/transaction.py", line 371, in inner
    return func(*args, **kwargs)
  File "/usr/lib/hue/apps/jobbrowser/src/jobbrowser/views.py", line 82, in decorate
    return view_func(request, *args, **kwargs)
  File "/usr/lib/hue/apps/jobbrowser/src/jobbrowser/views.py", line 237, in single_job
    failed_tasks = job.filter_tasks(task_states=('failed',))
  File "/usr/lib/hue/apps/jobbrowser/src/jobbrowser/yarn_models.py", line 268, in filter_tasks
    return [Task(self, task) for task in self.api.tasks(self.id).get('tasks', {}).get('task', [])
AttributeError: 'NoneType' object has no attribute 'get'