But what would happen in case the mempool is empty and there are no unconfirmed transactions propagated on the bitcoin network (basically no transactions in the block to be mined) .
Even with no transactions, miners would likely continue to mine for a number of reasons:
1 - They will want to unlock their previous block rewards, which are locked for 100 blocks.
2 - Miners and mining pools hold a significant amount of bitcoin. This is especially true of pools such as Binance pool. It is in their own interests to continue to secure the network so their own holdings don't lose value.
3 - We already have a number of protocols, merged mining, and other things of value which are based on bitcoin blocks being found. See for example the RSK OP_RETURN output in almost every block which is currently found. In the future there will likely be many more such projects, and so miners can gain value from elsewhere by mining bitcoin.