When using query.get_default_bone_pivot in an animation in a resource pack, it is impossible to tell whether an output of 0 is due to an invalid call or a valid call which resulted in 0. This is because an invalid call also returns 0, when it should be returning "null".

If this were fixed, it would provide resource packs a way to avoid the problem described in my comment on MCPE-126660: "On some devices, Minecraft references the cape geometry instead of the player geometry when running q.get_default_bone_pivot, which fails because the left and right arms are not listed in this geometry."

If packs could distinguish between a valid 0 output and a 0 output due to an invalid call, we could detect when Minecraft incorrectly queries cape geometry and use a fallback "default" value instead. Some skin types have valid bone pivots of 0, so detecting a 0 output is not sufficient to differentiate between valid and invalid output.

To Reproduce

1. Apply the attached sample resource pack: invalid_vs_valid_query_tester.mcpack
2. Understand the sample resource pack: 
   1. Screenshot:
      1. 
   2. The left leg walking animation will query an invalid bone: "i_do_not_exist".
   3. The right leg walking animation will query a valid bone: "rightleg", and the requested pivot value is 0 in the wide-arm custom skin model.
      1. 
   4. The null coalescing operator provides a default value for both legs. If a query fails, the default value should take its place. (See Expected Result)
   5. The default value should raise the leg above the player's head to indicate a problem with the query.
   6. When the player starts walking:
   7. The left leg query will fail, as i_do_not_exist is not a real bone.
   8. The right leg query will succeed, as rightleg is a real bone.
   9. The built-in null coalescing value should be used for the left leg, raising it above the player's head, but should not be used for the right leg, as the query was successful.
   10. The leg rotation while walking has been made more dramatic to prove the animation is working.
3. Ignore the content errors on the main menu and pause menu. These are not related to the issue. The animation will work fine in-game, as seen in Step 7.
4. Enter a world.
5. Switch to third-person back camera.
6. Start walking.
7. Observe the dramatic swinging, indicating the animation has been applied successfully.
8. Observe that both legs are in the normal position below the body.
9. Notice that this means our failed query was not processed as invalid output (null), but as a valid 0.
10. Notice this means that valid and invalid 0s cannot be distinguished from this query's output.

Expected Result

An invalid query.get_default_bone_pivot call returns "null", so the null coalescing operator can be used to configure a fallback value in invalid cases:

(variable.pivot = query.get_default_bone_pivot('i_do_not_exist',2); return (variable.pivot ?? 30.0); 

This line would use the value returned by the query unless it is invalid, in which case the 30.0 value would be used instead. Minecraft includes a MoLang "fixer" which processes "null" values as 0 for math already, so this will not break existing packs.

Observed Result

An invalid query.get_default_bone_pivot call returns 0, in the same way as a valid call. The null coalescing operator is unusable. Adding a "fallback value" is impossible because you cannot distinguish between a valid output of 0 and an invalid call.

To Fix

Make query.get_default_bone_pivot return "null" instead of 0 for invalid calls.