add blank material files

cstrike/pbr/models/pi_tree.mdl/pi_tree.mdl.mat

@@ -0,0 +1,8 @@
+
+{
+"for" "#models/pi_tree/pi_tree.mdl"
+//"basecolor_map" "pi_tree.png"
+//"normal_map" "pi_tree_normal.png"
+//"metal_map" "pi_tree_metal.png"
+//"roughness_map" "pi_tree_roughness.png"
+}