Full will generally be more efficient than empty, as the compressor
cycles will be longer and less frequent (takes longer to warm up and also to cool back down). Which means you get more cooling
time for each pump-down cycle the compressor goes through, leading to slightly better efficiency.
However, if the fridge isn't full, lining the sides with additional insulation
to take up some volume and reduce heat loss would be better than adding more food
(to slow heating
and cooling). But adding more contents is likely more practical in most cases.
In most cases, the full vs empty difference in efficiency won't be massive, so I wouldn't go crazy worrying about it.