Update FBXMeshGeometry.cpp (#5624)

Some FBXs do not have "Materials" information, which can cause parsing errors

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

code/AssetLib/FBX/FBXMeshGeometry.cpp

@@ -644,10 +644,12 @@ void MeshGeometry::ReadVertexDataMaterials(std::vector<int>& materials_out, cons
         return;
     }
 
+    if (source["Materials"]) {
         // materials are handled separately. First of all, they are assigned per-face
         // and not per polyvert. Secondly, ReferenceInformationType=IndexToDirect
         // has a slightly different meaning for materials.
         ParseVectorDataArray(materials_out, GetRequiredElement(source, "Materials"));
+    }
 
     if (MappingInformationType == "AllSame") {
         // easy - same material for all faces