[BLOCKING LIVE CONTENT] Entities defined in minecraft:addrider do not get added without a namespace

Build:
Release 853103

Summary:
Using the "minecraft:addrider" behaviour no longer works if the entity being added doesn't have the namespace before it.

Repro Steps:
1) Download Santa's Sleigh Ride
2) Create a new world using the Santa's Sleigh Ride template
3) Note the Assert popup
4) Trying to start the minigame will cause the player to get stuck on top of the snow globe as the skeleton cannot ride on top of the player.

Observed Results:

"minecraft:addrider": {
          "entity_type": "skeleton"
}

This causes an Assert where previously it didn't, meaning the map does not function as intended.

Expected Results: The behaviour should work with or without the namespace for the sake of backwards compatibility.

Impact: This breaks both Santa's Sleigh Ride and Block & Seek. Although it would be easy to fix on our end, this will impact users who already have the map and require an update for all players. If it worked previously, then it should continue to work now.