Summoning a Pillager with {} or any other NBT as long as you use brackets will spawn the pillager without its arms.

This only happens to Pillagers.

Spawning a pillager using /summon pillager ~ ~ ~ works like normal
Spawning a pillager using /summon pillager ~ ~ ~ {} removes the pillagers arms. It doesn't matter what data you put inside the brackets, but the pillager will spawn without it's arms. It will not attack you either, because it can't do that without it's arms.