Use MObject::hasObj to test for shadingEngine nodes
Use MObject::hasObj to test for shadingEngine nodes

MFnSet::hasObj() would return true even for regular sets. This was
causing material assignment logic to incorrectly thinks that a material
assignment was done.

sideeffects/HoudiniEngineForMaya,sideeffects/HoudiniEngineForMaya,sideeffects/HoudiniEngineForMaya