Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/repo/archiver.go

@@ -107,7 +107,7 @@ func GetRepoArchiver(ctx context.Context, repoID int64, tp ArchiveType, commitID
 	if has {
 		return &archiver, nil
 	}
-	return nil, nil
+	return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 }
 
 // ExistsRepoArchiverWithStoragePath checks if there is a RepoArchiver for a given storage path

models/repo/fork.go

@@ -49,7 +49,7 @@ func GetUserFork(ctx context.Context, repoID, userID int64) (*Repository, error)
 		return nil, err
 	}
 	if !has {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	return &forkedRepo, nil
 }

models/repo/repo.go

@@ -878,7 +878,7 @@ func IsRepositoryModelExist(ctx context.Context, u *user_model.User, repoName st
 // non-generated repositories, and TemplateRepo will be left untouched)
 func GetTemplateRepo(ctx context.Context, repo *Repository) (*Repository, error) {
 	if !repo.IsGenerated() {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil for non-generated repositories
 	}
 
 	return GetRepositoryByID(ctx, repo.TemplateID)

models/repo/topic.go

@@ -257,7 +257,7 @@ func DeleteTopic(ctx context.Context, repoID int64, topicName string) (*Topic, e
 	}
 	if topic == nil {
 		// Repo doesn't have topic, can't be removed
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the topic does not exist
 	}
 
 	return db.WithTx2(ctx, func(ctx context.Context) (*Topic, error) {