[unity] Exception when SkeletonRenderSeparator causes null material with mask materials

[HaraldCsaszar]

When mask materials are used and SkeletonRenderSeparator leads to an empty mesh with null material, mask material creation throws an exception when creating the mask material said null material.

Reported on the forum:
https://en.esotericsoftware.com/forum/d/25965-problem-when-assigning-skeletondata-to-skeletonanimation-at-runtime
Repro project sent via email:
FMfcgzGxSbvNCGnFlpFwwlTPHlqXghxB