Federation breaks when user arrives without referrer

[jeriox]

Traceback (most recent call last):
  File "/home/staghios/.local/lib/python3.11/site-packages/django/utils/datastructures.py", line 84, in __getitem__
    list_ = super().__getitem__(key)
            ^^^^^^^^^^^^^^^^^^^^^^^^

During handling of the above exception ('referrer'), another exception occurred:
  File "/home/staghios/.local/lib/python3.11/site-packages/ephios/plugins/federation/views/api.py", line 80, in get
    else FederatedGuest.objects.get(url=self.request.GET["referrer"])
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/staghios/.local/lib/python3.11/site-packages/django/utils/datastructures.py", line 86, in __getitem__
    raise MultiValueDictKeyError(key)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^