AY-7320 Skip meshes without faces in explicit face set member assignments

[BigRoy]

Changelog Description

1. Skip meshes without faces in explicit face set member assignments
2. Consider only doing the face assignment conversion on meshes actually in the export, instead of all that happen to be in the instance

Additional review information

• Avoids a No object matches name error when trying to force face shader assignments for Write Face Sets if a mesh has no faces at all.
• Also fixes the forced face set assignments to process all meshes in the instance, now it only includes e.g. meshes that are inside the out_SET for animation instances.

Testing notes:

1. Export a mesh without faces (yeah, that'd still be a problem!) with Write Face Sets with this conversion enabled.
2. It should not error on our code.

[moonyuet]

Code looks good for me. Did the test with published in maya and imported in unreal. The face sets for shader still kept in Unreal as expected.