Handle stoken being the empty string.

For whatever reason some users were getting this which was causing this code
to fail.

etebase_fastapi/routers/collection.py

@@ -473,7 +473,7 @@ def item_bulk_common(
     with transaction.atomic():  # We need this for locking the collection object
         collection_object = queryset.select_for_update().get(uid=uid)
 
-        if stoken is not None and stoken != collection_object.stoken:
+        if stoken and stoken != collection_object.stoken:
             raise HttpError("stale_stoken", "Stoken is too old", status_code=status.HTTP_409_CONFLICT)
 
         data.validate_db()

etebase_fastapi/stoken_handler.py

@@ -12,7 +12,7 @@ StokenAnnotation = t.Any
 
 
 def get_stoken_obj(stoken: t.Optional[str]) -> t.Optional[Stoken]:
-    if stoken is not None:
+    if stoken:
         try:
             return Stoken.objects.get(uid=stoken)
         except Stoken.DoesNotExist: