loot_table condition for player_generates_container_loot trigger is not optional

The bug

The loot_table condition used by the player_generates_container_loot advancement trigger is required and non optional. This is inconsistent with other advancement conditions, which are all optional. It also means that you cannot detect when the player generates any loot.

How to reproduce

1. Create the following advancement in a data pack

   {
     "criteria": {
       "requirement": {
         "trigger": "minecraft:player_generates_container_loot"
       }
     }
   }

2. Reload the game
   The advancement did not load and there is an error in the log

   Parsing error loading custom advancement test:gen_loot: Missing loot_table, expected to find a string