Fix possible nil description of pull request when migrating from CodeCommit (#34541) (#34550)

Backport #34541 by @lunny

Fix #34320

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/migrations/codecommit.go

@@ -180,11 +180,15 @@ func (c *CodeCommitDownloader) GetPullRequests(ctx context.Context, page, perPag
 			continue
 		}
 		target := orig.PullRequestTargets[0]
+		description := ""
+		if orig.Description != nil {
+			description = *orig.Description
+		}
 		pr := &base.PullRequest{
 			Number:     number,
 			Title:      *orig.Title,
 			PosterName: c.getUsernameFromARN(*orig.AuthorArn),
-			Content:    *orig.Description,
+			Content:    description,
 			State:      "open",
 			Created:    *orig.CreationDate,
 			Updated:    *orig.LastActivityDate,