providers/oauth2: make exp optional on jwt client_credentials flow

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

authentik/core/tasks.py

@@ -60,6 +60,8 @@ def clean_temporary_users(self: MonitoredTask):
     messages = []
     deleted_users = 0
     for user in User.objects.filter(**{f"attributes__{USER_ATTRIBUTE_GENERATED}": True}):
+        if USER_ATTRIBUTE_EXPIRES not in user.attributes:
+            continue
         delta: timedelta = _now - datetime.fromtimestamp(
             user.attributes.get(USER_ATTRIBUTE_EXPIRES)
         )