Previously, in 4.2, the `has_face_set` method always returned `true` for
whether or not a given vert had a given face set with dyntopo enabled.
This logic was updated to be more complete in 4.3, but the fallback
logic for when a mesh didn't have any face set data was not.
To fix this, if the corresponding BMesh doesn't have the face set
attribute, we check whether or not thet requested face set is
`SCULPT_FACE_SET_NONE`
Pull Request: https://projects.blender.org/blender/blender/pulls/132523
3c5b2c3b9f