If you don't care about the Key value (which may have been assigned, ie you had a null Long id), then you can ignore the return value.
Not directly in your question but needs to be mentioned: Using the automatic id generator inside of a transaction is dangerous and should not be done, because it's not an idempotent operation. It's possible that you may get a transaction retry even though the transaction succeeded, and the retry will create a new entity. Always assign ids outside of the transaction.
I will go further and say that you should not use automatic assignment of ids at all. Explicitly generate them with the allocator when you construct your POJOs.