Miners are (and always have been) allowed to mine blocks containing zero transactions (i.e. ~0kB, not including necessities). It's us nodes that choose to reject/accept (of course there is Venn overlap).

This was common until block congestion actually started encouraging senders to include transaction fees; these [still!-]voluntarily fees represent a healthy addition to the blockreward, so most miners incur the minimal overhead for much-greater profit.

Yet zero-transaction blocks continue to be mined (albeit less-frequently).