Refine coding

* Adjust a few things per Hoikas' suggestion

korman/exporter/material.py

@@ -701,9 +701,8 @@ class MaterialConverter:
             layer.texture = pl_env.key
 
     def export_cubic_env(self, bo, layer, texture):
-        if texture.image == None:
+        if texture.image is None:
-            raise ExportError("CubeMap '{}' has no cube image!".format(texture.name))
+            raise ExportError(f"CubeMap '{texture.name}' has no cube image!")
-        else:
         width, height = texture.image.size
 
         # Sanity check: the image here should be 3x2 faces, so we should not have any