Fix panic when migrating a repo from GitHub with issues (#25246)

Fix #25245. Regression of #23946.

Author: wolfogre

services/migrations/github.go

@@ -487,7 +487,7 @@ func (g *GithubDownloaderV3) GetIssues(page, perPage int) ([]*base.Issue, bool,
 			Updated:      issue.GetUpdatedAt().Time,
 			Labels:       labels,
 			Reactions:    reactions,
-			Closed:       &issue.ClosedAt.Time,
+			Closed:       issue.ClosedAt.GetTime(),
 			IsLocked:     issue.GetLocked(),
 			Assignees:    assignees,
 			ForeignIndex: int64(*issue.Number),