During cache culling, diskcache can have `None` keys that get unhandled

[rtibbles]

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This might just be an issue with diskcache that would be resolved by upgrading - but I think we're on the newest version that still supports Python 3.6.

  File "/exports/projectpool/fle/kolibri-ai/kolibri/utils/cli.py", line 199, in invoke
    initialize(**get_initialize_params())
  File "/exports/projectpool/fle/kolibri-ai/kolibri/utils/main.py", line 303, in initialize
    _post_django_initialization()
  File "/exports/projectpool/fle/kolibri-ai/kolibri/utils/main.py", line 233, in _post_django_initialization
    process_cache.cull()
    ^^^^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/django/utils/functional.py", line 246, in inner
    self._setup()
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/django/utils/functional.py", line 382, in _setup
    self._wrapped = self._setupfunc()
                    ^^^^^^^^^^^^^^^^^
  File "/exports/projectpool/fle/kolibri-ai/kolibri/core/utils/cache.py", line 13, in __get_process_cache
    return caches["process_cache"]
           ~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/django/utils/connection.py", line 62, in __getitem__
    conn = self.create_connection(alias)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/django/core/cache/__init__.py", line 44, in create_connection
    return backend_cls(location, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/djangocache.py", line 31, in __init__
    self._cache = FanoutCache(directory, shards, timeout, **options)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/fanout.py", line 43, in __init__
    self._shards = tuple(
                   ^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/fanout.py", line 44, in <genexpr>
    Cache(
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/core.py", line 456, in __init__
    sql = self._sql_retry
          ^^^^^^^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/core.py", line 652, in _sql_retry
    sql = self._sql
          ^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/core.py", line 648, in _sql
    return self._con.execute
           ^^^^^^^^^
  File "/home/jamalex/.virtualenvs/kolibri-ai/lib/python3.12/site-packages/diskcache/core.py", line 641, in _con
    if key.startswith('sqlite_'):
       ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'startswith'```