It means they missed out on all the txn fees that would be in a full block.
Discussion
So they just get the blow reward but no extras. Got it. Is that a common thing?
Yeah it happens fairly frequently. In theory it should happen exactly proportionally to the amount of time it takes to download the block template to the average block time. So as an example if it takes 4 seconds for the block to download to miners and the average block time is 10 minutes then on average 0.67% of blocks found will be empty.
Yes, but if they had discarded the empty block template that they HAD found a valid hash on, and then started trying to find a valid hash for the template that included all the transactions, they very likely would not have been lucky enough to find it, and therefore lost out on not only the transaction fees, but the block reward as well.
Block reward without transaction fees beats risking not getting either any day.