Fix not handled errors when op don't have polygons

Thanks, Andreas Block for pointing it.

Author: gr4ph0s

scripts/04_3d_concepts/modeling/read_write_normal_tag_r13.py

@@ -100,6 +100,10 @@ def main():
     if not isinstance(op, c4d.PolygonObject):
         raise TypeError("op is not a c4d.PolygonObject.")
 
+    # Checks if the selected polygon object has some polygons
+    if op.GetPolygonCount() == 0:
+        raise ValueError("The the selected object, don't have any polygon to host normal data.")
+        
     # Creates a normal tag
     tag = CreateNormalTag(op)
 
@@ -123,4 +127,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    main()