Missing URL to Wikipages triggers HTTP 500

[Hetti]

Pull Request #239 introduced a bug, that it will throw a HTTP 500 error if no wiki page is provided.

How to Reproduce:

1. go to https://metalab.at/calendar/
2. be logged in
3. create an event with with no URL for wikipage provided
4. 500 🎉

Error (exceprt) as seen below

  File "/usr/local/lib/python3.11/site-packages/django/contrib/auth/d>
    return view_func(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/code/cal/views.py", line 195, in update_event
    if event_form.is_valid():
       ^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/django/forms/forms.py>
    return self.is_bound and not self.errors
                                 ^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/django/forms/forms.py>
    self.full_clean()
  File "/usr/local/lib/python3.11/site-packages/django/forms/forms.py>
    self._clean_form()
  File "/usr/local/lib/python3.11/site-packages/django/forms/forms.py>
    cleaned_data = self.clean()
                   ^^^^^^^^^^^^
  File "/code/cal/forms.py", line 31, in clean
    wikipage, _ = re.subn(r'(^http(s)://metalab.at/wiki/|\ |\%|\&)', >
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
  File "/usr/local/lib/python3.11/re/__init__.py", line 196, in subn
    return _compile(pattern, flags).subn(repl, string, count)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes-like object, got 'NoneType'