Closes https://github.com/go-gitea/gitea/issues/39414
`CommentList.loadAssignees` set a Ghost assignee on every comment
without an `AssigneeID` whenever another comment in the same batch had
one. Since https://github.com/go-gitea/gitea/pull/38413 the timeline
prefers `Assignee` over `AssigneeTeam`, so team review requests, from
CODEOWNERS or added manually, rendered as "Ghost" whenever the timeline
also had a user assignee or user review request. Skip comments without
an `AssigneeID`.
Co-authored-by: silverwind <me@silverwind.io>