fix: OpenIDConnectFrontend: check for empty db_uri (#420)

With IdentityPython/pyop#44 merged, OpenIDConnectFrontend init fails
when `db_uri` is not set, as `StorageBase.type` now throws a `ValueError`
for db_uri values that do not match one of the recognised storage types
(including when `db_uri` is `None`).

Fix this by guarding the `StorageBase.type` with a pythonic test
whether `db_uri` was provided.

Same test already guards `StorageBase.from_uri`, add it also
to the `StorageBase.type` call made to determine `self.stateless`.

src/satosa/frontends/openid_connect.py

@@ -72,7 +72,7 @@ def __init__(self, auth_req_callback_func, internal_attributes, conf, base_url,
         )
 
         db_uri = self.config.get("db_uri")
-        self.stateless = StorageBase.type(db_uri) == "stateless"
+        self.stateless = db_uri and StorageBase.type(db_uri) == "stateless"
         self.user_db = (
             StorageBase.from_uri(db_uri, db_name="satosa", collection="authz_codes")
             if db_uri and not self.stateless