Backport #38112
## Problem
`GET /repos/{owner}/{repo}/times` and `GET
/repos/{owner}/{repo}/issues/{index}/times` crash with a nil pointer
dereference when the `user` query filter names a user that does not
exist.
## Root cause
In `ListTrackedTimes` and `ListTrackedTimesByRepository`, the
`IsErrUserNotExist` branch sends the 404 but is missing a `return`, so
execution falls through to `opts.UserID = user.ID` with a nil `user`.
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Co-authored-by: Pycub <iamsokhandan@gmail.com>
