There are no inputs and outputs in Ethereum, just state changes and balances. Therefore, mining rewards don't have a transaction hash since they are not a transaction. 

Each block contains the miner's address, and when the block is published, the miner just gets 5 more Ether added to their balance, without the need for a transaction.

See [this question][1] for more about UTXOs vs Balances


  [1]: http://ethereum.stackexchange.com/q/326/131