Sometimes when a teacher is adding a contest, the contest is created with an empty `controller_name`, which causes `controller` to be None, which causes tons of AttributeErrors on 2/3 of pages, including the main portal / landing page.

Stacktrace from a sanity check I added to diagnose this issue:

https://sentry.io/the-sio2-project/szkopul/issues/351545304/

{code}
ValueError: Tried to save contest with empty controller
  File "oioioi/contests/models.py", line 79, in save
    raise ValueError("Tried to save contest with empty controller")
  File "oioioi/contests/forms.py", line 76, in save
    instance.save()
  File "oioioi/base/admin.py", line 333, in add_view
    return model_admin.add_view(request, form_url, extra_context)
...
(12 additional frame(s) were not displayed)

ValueError: Tried to save contest with empty controller
{code}

Probable cause:
If there are no rounds specified, this code saves the contest even if it's not supposed to (commit=False):
https://github.com/sio2project/oioioi/blob/master/oioioi/contests/forms.py#L73-L82

Git blame suggests it's been there since the very beginning, I have on idea why this issue didn't appear before.