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Alembic export: support simple child hairs (Fix T51144)

Simple child hairs don't have a face index number assigned, so the
call to dm->getTessFaceData(dm, num, CD_MFACE) would cause a crash. To
work around this, UV and normal vectors are copied from the parent
hair.

I've also removed an unnecessary call to dm->getTessFaceArray(dm);

Reviewers: kevindietrich

Differential Revision: https://developer.blender.org/D2638