Fixed _url_join when None in parameters

Change-Id: I342f91085ae689c29175bc8d5d4bd357b2408d02
diff --git a/rsync_url.py b/rsync_url.py
index 968a627..83061ff 100644
--- a/rsync_url.py
+++ b/rsync_url.py
@@ -158,7 +158,7 @@
             while url.endswith(os.path.sep):
                 url = url[:-1]
 
-        subs = os.path.sep.join(suburls).split(os.path.sep)
+        subs = os.path.sep.join([_ for _ in suburls if _]).split(os.path.sep)
         subs = [_ for _ in subs if _]
 
         result = re.sub(r'^//', r'/', os.path.sep.join([url, ] + subs))