Here's a little known fact about Congresscritters and their empty seats.
As you've probably seen on C-SPAN, congressmen have name plates in front of their seats when they are having committee hearings. Before the advent of CSPAN and cameras in the committee rooms, the name plates stayed in place even if the congressman wasn't present during a committee hearing. Once pictures started getting out of name plates in front of empty seats during important hearings, and these pictures and videos became political jabs by opponents, congress started having interns, pages, and staffers remove the nameplates if a congressman isn't present.
You'll sometimes see this on C-SPAN; if you ever catch a congressman get up and leave during a hearing, someone will come up almost immediately and remove the name plate.
That's a long way of saying... Trump was trolling them hard with this picture.