What Is An Empty Block?

In Bitcoin, empty blocks (or blocks with just 1 transaction) occur from time to time.  In the early days of bitcoin, empty blocks were very common as there were no transactions to be mined. Bitcoin has grown since, though, and there are thousand of unverified transactions in the mempool every single moment. But empty blocks or blocks with very few transactions continue to appear.

There are various explanations why this happens but the most logical one is related to mining strategies as well as optimizations.

Most mining pools start mining as soon as they have the previous block header. If they find a nonce before they have verified all the transactions it holds, they send an "empty block".  In that case, the empty block contains just 1 transcation which is the coinbase (reward transaction).

Also mining pools do some kind of optimizations on broadcasting new jobs just after another blocks is found. Some do SPV mining (mining on top of unvalidated block), other just validate previous block and save time by skipping mempool.

Some miners might choose to include fewer transactions to get their new block out to the rest of the peers on the network as soon as possible. If a block has few transactions, it is much smaller in size. This gives other miners less time to solve a block while the smaller one is propagating, and it also increases the chance that block to be accepted by the majority of the miners even if someone else solved a block slightly before it.

Another explanation for empty blocks is that the bitcoin network is being spammed. When this happens transactions that need to be processed have to wait till the next block.

How often does a block have only one transaction?

In the past it has been computed that almost 1,5% of blocks mined are empty (only coinbase).