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/BlockThere should be some rules , for example , at least xxx trans or xx% of all unconfirmed trans
No actually there is not such rules about block should contain xxx trans or x% of unconfirmed transactions having coinbase only transaction make it a valid block according to the bitcoin wiki above.
Why you think there should be such rules? If two subsequent block get mined within few sec difference and there is not much transaction within that small time fraction than this type of block with only one transaction can form.