set_fireworks "mode" is required even when "explosions" is optional

In the set_fireworks loot item function, the "mode" field is always required, even when it is only used to apply the optional "explosions" lists.

How to reproduce

1. Create a data pack with the following item modifier, I expect it to set the flight duration to 2 and keep the explosions the same

   {
     "function": "set_fireworks",
     "flight_duration": 2
   }

   {
     "function": "set_fireworks",
     "flight_duration": 2
   }

2. ❌ The item modifier is invalid

3. Add the "mode" field

   {
     "function": "set_fireworks",
     "mode": "append",
     "flight_duration": 2
   }

   {
     "function": "set_fireworks",
     "mode": "append",
     "flight_duration": 2
   }

4. This makes the item modifier valid again and it behaves as expected. (Internally it appends an empty list of explosions to the existing explosions)