First, I doubt that this optimization actually changes anything measurable, but if you want you can always replace that line by "if part.startswith(sep)" (also solving the original issue).

Additionally, note that `parse_parts` (the only function to call `splitroot` already (and redundantly) checks that `part` is truthy before calling `splitroot`.  There are a couple of other "optimizations" in the same function (`if x and x != '.'`, `if rel and rel != '.'`) for which I'd be surprised if they truly mattered.