Given the example advancement, placed in world/data/advancements/custom/test.json:

{
    "criteria": {
        "findme": {
            "trigger": "minecraft:impossible"
        }
    },
    "rewards": {
        "loot": ["custom:test"]
    }
}

And the following commands to grant and revoke it at will (see MC-115064 for namespace issue):

/advancement grant @p minecraft:custom/test
/advancement revoke @p minecraft:custom/test

Assuming the loot table exists, no items are provided and no errors occur. If the loot table doesn't exist, an error will occur as expected.