core: fix PropertyMapping context not being available in request context

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

blueprints/system/providers-proxy.yaml

@@ -14,8 +14,11 @@ entries:
       expression: |
         # This mapping is used by the authentik proxy. It passes extra user attributes,
         # which are used for example for the HTTP-Basic Authentication mapping.
+        session_id = None
+        if "token" in request.context:
+            session_id = request.context.get("token").session_id
         return {
-            "sid": token.session_id,
+            "sid": session_id,
             "ak_proxy": {
                 "user_attributes": request.user.group_attributes(request),
                 "is_superuser": request.user.is_superuser,