You can answer with "khong co gi". Young people in the south also use "khong co ji", but that is dialect. If you want to be polite, you can also put "vang" or "da" (south) before it. Sorry, I still haven't downloaded VietKey, so no diacritical signs.

You can also with: "Không có gì" or "Không có chi" (Southern dialect) - No problem Or "Chuyện nhỏ ấy mà" or "Chuyện vặt ấy mà" - It's not a big deal It's more casual but not recommended in formal situation.