Fix empty owner tokens start_index error

[YellingOilbird]

When the user's token_set is empty this method panics now in case of use

{
        ...
        from_index: None,
        limit: None,
}

But i propose for an empty list returned in this situation

[austinabell]

that's valid for sure. I wonder why there is the require below is there at all. Going to pull this in since it's just for a view call and this is minimally changing from previous

[austinabell]

thank you!