fix: handle missing discussion settings when importing old courses

[Agrendalath]

This fixes the following error when importing an old course:

Task cms.djangoapps.contentstore.tasks.import_olx[ID]: Course import course-v1:ID: DiscussionsConfiguration sync failed: 'provider_type'

# The traceback is hidden by default, so it needs to be manually re-raised to see the actual error:
Traceback (most recent call last):
   File "/openedx/venv/lib/python3.11/site-packages/celery/app/trace.py", line 453, in trace_task
     R = retval = fun(*args, **kwargs)
                  ^^^^^^^^^^^^^^^^^^^^
   File "/openedx/edx-platform/cms/djangoapps/contentstore/tasks.py", line 718, in import_olx
     sync_discussion_settings(courselike_key, user)
   File "/openedx/edx-platform/cms/djangoapps/contentstore/tasks.py", line 458, in sync_discussion_settings
     and not course.discussions_settings['provider_type'] == Provider.OPEN_EDX
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
 KeyError: 'provider_type'

Testing instructions

This is a trivial change, so simply check that the tests are passing.

[openedx-webhooks]

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