what's inside those empty blocks? (i mean, what is "computationally expensive" from an empty item)
Empty blocks still contain data. They aren't devoid of data, they simply do not have transactions other than the coin generation transaction (known as the coinbase transaction) in them. Since an empty block does not contain any transactions from the Bitcoin network, it is considered to be empty. Empty blocks are still computationally expensive because miners still have to produce a Proof of Work. They still have block headers which are 80 bytes and have all of the fields that non-empty blocks do. They still have a list of transactions; but that list just contains one transaction, the coinbase transaction, which pays the miner the block subsidy.
how can a system creates blocks that are empty and have no use (not for transactions use)
Empty blocks still have a use. They aren't completely useless. Firstly, they extend the blockchain adding additional work to the blockchain. The transactions that a block contains does not effect how much work needs to be done to mine the block, so empty blocks strengthen the security of the blockchain by adding to the cumulative work gone into the blockchain.
Empty blocks also introduce new coins to the Bitcoin system. They still pay miners for the work that they did, so new coins are generated and paid to the miner of that block. This is part of the coin generation transaction included in the block itself as the only transaction.
Additionally, when there are no transactions, the network still needs to move on. There still needs to be blocks mined, so empty blocks must be allowed in order for the blockchain to continue.
what happen if there are transactions example like sending btc to other address but nobody mining it (verifying it through mining), will the transactions ever get through to the destined address?
If nobody ever includes a transaction in a block, then the transaction is never considered to be final. It will still appear in users' wallets as unconfirmed and they will see the coins. They can even spend the outputs of that transaction, but creating transactions from unconfirmed transactions is not advised. Such a transaction would also be unconfirmed so long as the first unconfirmed transaction remains unconfirmed.