Fix: Sphinx extension on Python3

This patch is fixing the WSME Sphinx extension to work with Python 3.
The patch just checks the return value from the find_service_path()
method before it tries to do anything with it.

Change-Id: I5a4cfa42a8768ce35a1c1a999431e842c7dde020
Closes-Bug: #1657435

wsmeext/sphinxext.py

@@ -361,6 +361,8 @@ class ServiceDocumenter(autodoc.ClassDocumenter):
 
     def format_name(self):
         path = find_service_path(self.env, self.object)
+        if path is None:
+            return
         return '/' + '/'.join(path)