When setting a spawner's type to null using _{CreatureSpawner#setSpawnedType} it's set properly and can be read from the state later, but calling _{BlockState#update} doesn't seem to update the actual spawner in the world.
Tested with:

CreatureSpawner spawner = (CreatureSpawner) block.getState();
spawner.setSpawnedType(null);
player.sendMessage("Set to: " + spawner.getSpawnedType()); // Outputs null
spawner.update();

Setting it to any entity type seems to work as expected.
 
Not sure if this counts as Minor or Major, so marked it as Minor for now.