The bug

If a set_attributes function contains an undefined attribute, the item stack will try to put null to tag.AttributeName and throws java.lang.NullPointerException.

How to reproduce

1. /loot give @s loot mc-177253:test

   → An unexpected error occurred trying to execute that command (java.lang.NullPointerException)

• data/mc-177253/loot_tables/test.json

  {
    "type": "minecraft:generic",
    "pools": [
      {
        "rolls": 1,
        "entries": [
          {
            "type": "minecraft:item",
            "name": "minecraft:stone",
            "functions": [
              {
                "function": "minecraft:set_attributes",
                "modifiers": [
                  {
                    "name": "mc-177253",
                    "attribute": "mc-177253:undefined",
                    "operation": "addition",
                    "amount": 0.0,
                    "slot": "mainhand"
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  }