FBX Import: skip invalid custom enum properties

This was (correctly) asserting before, now handle this more gracefully
and just skip (and warn about this) a custom property that has an
invalid value set.

Seems there are a couple of exporters out there that do this wrong, I
think this tradeoff can be made though.

Fixes T91062, T81657, T83501, T86595

Maniphest Tasks: T91062, T86595, T83501, T81657

Differential Revision: https://developer.blender.org/D12354