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

models/issues/comment_test.go

@@ -58,6 +58,20 @@ func TestLoadAssigneeUserAndTeam_DeletedTeamBecomesGhostTeam(t *testing.T) {
 	assert.EqualValues(t, -1, comment.AssigneeTeam.ID)
 }
 
+func TestCommentListLoadAttributesMixedAssignees(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+	comments := issues_model.CommentList{
+		{AssigneeTeamID: 8},
+		{AssigneeID: 999999},
+	}
+
+	assert.NoError(t, comments.LoadAttributes(t.Context()))
+	assert.Nil(t, comments[0].Assignee)
+	assert.Zero(t, comments[0].AssigneeID)
+	assert.Equal(t, user_model.GhostUserID, comments[1].AssigneeID)
+	assert.Equal(t, user_model.GhostUserID, comments[1].Assignee.ID)
+}
+
 func Test_UpdateCommentAttachment(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())