I always thought that FAs do the safety demonstration if there are no screens throughout the cabin, and they do in in both cabins in the case of the 738.

Was on an early morning flight out of JFK in first, just mindlessly playing on my phone while the safety demonstration was going on when I noticed there was no FA doing it for the first cabin. There was one right behind me doing it at the bulkhead for econ.

Just found it odd. Maybe something came up in front and the other FA was dealing with it.