fix: userinfo: avoid indexing by None user_id in stateless mode (#46)

In stateless mode, `user_id` passed to `get_claims_for` is always `None`.

When no claims are available and userinfo is thus also `None`, `user_id`
is used as index to self._db, but that triggers a `KeyError: None`.

As `create_access_token` populates `user_info` field in `authz_info`
only of `user_info` is pythonically `True`, `user_info` passed here ends
up being `None`, not empty dict `{}`.

As the `Userinfo` class does not have access to `Provider.stateless`,
the easiest fix is to make the db lookup conditional on user_id being
pythonically True - which corresponds with not being in stateltess mode.

Author: vladimir-mencl-eresearch

src/pyop/userinfo.py

@@ -31,6 +31,6 @@ def get_claims_for(self, user_id, requested_claims, userinfo=None):
         """
 
         if not userinfo:
-            userinfo = self._db[user_id]
+            userinfo = self._db[user_id] if user_id else {}
         claims = {claim: userinfo[claim] for claim in requested_claims if claim in userinfo}
         return claims