fix(issues): don't render team review requests as Ghost (#39416)

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>

models/issues/comment_list.go

@@ -201,6 +201,9 @@ func (comments CommentList) loadAssignees(ctx context.Context) error {
 	}
 
 	for _, comment := range comments {
+		if comment.AssigneeID <= 0 {
+			continue
+		}
 		comment.Assignee = assignees[comment.AssigneeID]
 		if comment.Assignee == nil {
 			comment.AssigneeID = user_model.GhostUserID