The mining isn't based on transactions, the mining is set to target a 10 minute block time, the difficulty changes based on how much mining capacity there is.
If a block takes less than 10 minutes the difficulty goes up. If it takes more the difficulty goes down.
Each block has a maximum capacity of 2500 transactions, which is why gas is used to prioritize who gets added in a block.
There have been many blocks with zero transactions.