update: add testcase for empty color.

Signed-off-by: Kim Kulling <[email protected]>

test/models/OBJ/empty_mat.mtl

@@ -4,7 +4,7 @@ newmtl
 Ns 0
 Ka 0.000000 0.000000 0.000000
 Kd 0.8 0.8 0.8
-Ks 0.8 0.8 0.8
+Ks 0
 d 1
 illum 2