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/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) {