a bitcoin block can be mined as long as there is at least one transaction in it, and that one transaction is the coinbase transaction containing the reward.

you can read the wiki about the specifications of a bitcoin block for more detailed information: https://en.bitcoin.it/wiki/Block

edit:
also you could easily use google to find these:
https://bitcoin.stackexchange.com/questions/34089/how-often-does-a-block-have-only-one-transaction
https://bitcoin.stackexchange.com/questions/24098/why-there-are-blocks-with-single-transaction-in-blockchain