Does it mean that at that time there was no unconfirmed transaction.
It simply means that at the time the block was built, the miner simply chose not to include any other transactions. It could simply be because the miner knew of the previous block but had not processed it yet so it did not know what transactions were included so none were included. Since it probably did not exhaust all of the possible nonces, the block remained empty since there was no need to have to rebuild the block to get a different hash.
Or it was manually pushed to get only the mandatory transaction?
What do you mean by "manually pushed"?