tmp (wokkel.rsm): raise RSMError when <after/> element is empty

src/tmp/wokkel/rsm.py

@@ -117,6 +117,8 @@ class RSMRequest(object):
             after = None
         else:
             after = unicode(after_elt)
+            if not after:
+                raise RSMError("<after/> element can't be empty in RSM request")
 
         try:
             max_elt = set_elt.elements(NS_RSM, 'max').next()