The bug

The world generation logs the warning "Empty or none existent pool : minecraft:empty" apparently for structures using the jigsaw block (currently the pillager outpost) when using minecraft:empty as pool.

The message should probably also be "Empty or non-existent pool: " ("non-existent" and no space between "pool" and the colon).

[20:31:38] [WorldGen-Worker-8/WARN]: Empty or none existent pool : minecraft:empty
[20:31:38] [WorldGen-Worker-8/WARN]: Empty or none existent pool : minecraft:empty
[20:31:38] [WorldGen-Worker-8/WARN]: Empty or none existent pool : minecraft:empty
[20:31:38] [WorldGen-Worker-8/WARN]: Empty or none existent pool : minecraft:empty
[20:31:38] [WorldGen-Worker-8/WARN]: Empty or none existent pool : minecraft:empty
...

How to reproduce

1. Locate a pillager outpost

   /locate Pillager_Outpost
   

2. Teleport to the outpost