Material set conversion routines need to be sensitive to optionally included material map #1507
Description
Currently, the material set conversion routines expect that there will only be a material map in the case of uni buffer element dominant (sparse by element) data. Therefore, a material map is created as needed. But the Blueprint specification allows for there to be material maps in all material set representations, so we must handle the case that a material map is provided and honor it, as it may be different from the default constructed map we create.