[unity] SkeletonDataAsset with 0 atlas assets throws confusing exception in build

[HaraldCsaszar]

Reported on the forum:
https://esotericsoftware.com/forum/d/26252-swapping-atlases-based-on-screen-resolution/21
SkeletonDataAsset with 0 atlas is expected to load without regions, but throws confusing exception instead when Spine/Special/HiddenPass shader is not found (not included in Always Included Shaders).