appview/indexer/repos: check `RepoStats` nullability

Fix appview segfault on new repo creation.
`RepoStats` can be empty. It is recommended to check it before
accessing.

Signed-off-by: Seongmin Lee <[e-mail]>

appview/indexer/repos/indexer.go

···
 }
 
 func makeRepoData(repo *models.Repo) *repoData {
+	var language string
+	if repo.RepoStats != nil {
+		language = repo.RepoStats.Language
+	}
 	return &repoData{
 		ID:          repo.Id,
 		RepoAt:      repo.RepoAt().String(),
···
 		Topics:      repo.Topics,
 		TopicsExact: repo.Topics,
 		Knot:        repo.Knot,
-		Language:    repo.RepoStats.Language,
+		Language:    language,
 	}
 }