Fix exception when no castxml is found with Python 3

Author: iMichka

pygccxml/utils/utils.py

@@ -53,7 +53,7 @@ def find_xml_generator(name="castxml"):
     else:
         path = _find_xml_generator_for_legacy_python(name)
 
-    if path == "":
+    if path == "" or path is None:
         raise(Exception("No c++ parser found. Please install castxml."))
     return path.rstrip(), name