I'll source the Bitcoin Wiki for the definition of mining,
https://en.bitcoin.it/wiki/Mining. The very first sentance of the page states:
Mining is the process of adding transaction records to Bitcoin's public ledger of past transactions.
Therefore, if you're not adding transactions to the blockchain (i.e. solving an empty block) then you are not mining.
Here's a few other mining definitions (emphasis added by me):
https://bitcoin.org/en/faq#what-is-bitcoin-miningMining is the process of spending computing power to process transactions, secure the network, and keep everyone in the system synchronized together.
http://www.investopedia.com/terms/b/bitcoin-mining.aspBitcoin mining is the process by which transactions are verified and added to the public ledger, known as the block chain, and also the means through which new bitcoin are released.
http://whatis.techtarget.com/definition/Bitcoin-miningBitcoin mining is the processing of transactions in the digital currency system, in which the records of current Bitcoin transactions, known as a blocks, are added to the record of past transactions, known as the block chain.
http://en.wikipedia.org/wiki/Bitcoin#MiningMining is a record-keeping service.[note 9] Miners keep the block chain consistent, complete, and unalterable by repeatedly verifying and collecting newly broadcast transactions into a new group of transactions called a block.
Empty blocks are not mining.
Until the block rewards subsidy goes away, an empty block still technically has one transaction: the coinbase which distributes the generated coins from the block reward.
So, while it isn't adding any value by including other transactions, it does, indeed technically fit the definition of mining.