[BUG] `useNbt: true` with an empty NBT string causes an error when you try to export.

[TheAfroOfDoom]

Describe the bug useNbt: true with an empty NBT string causes an error when you try to export.

To Reproduce Steps to reproduce the behavior:

1. Create a bone
2. Set Use NBT to true in the bone config
3. Click confirm (leave the NBT field empty)
4. Export model
5. See error

Expected behavior

Two things:

1. Gracefully handle an empty string (defaulting to "{}" probably)
2. Output a more meaningful error message / run NBT/JSON validation on the input early

Error Message

Expected value at position 0: <--[HERE]
Error: Expected value at position 0: <--[HERE]
    at Us.createError (<anonymous>:5961:142)
    at Object.readTag (<anonymous>:5964:4363)
    at NbtCompound.fromString (<anonymous>:5967:343)
    at NbtTag.fromString (<anonymous>:5961:51288)
    at $t.toNBT (<anonymous>:5967:153001)
    at hashRig (<anonymous>:6842:19685)
    at actuallyExportProject (<anonymous>:6842:22321)
    at async exportProject (<anonymous>:6847:68)

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• AJ 1.0.0 pre-4

Additional context