VOL-504 Fix exception in case of members=None in temporal state

Change-Id: I91296d9992b8d4951e7fbd54d7eb657d3b79dcdd
diff --git a/voltha/leader.py b/voltha/leader.py
index 62bf1d5..60d1e6c 100644
--- a/voltha/leader.py
+++ b/voltha/leader.py
@@ -226,7 +226,7 @@
                     returnValue(False)
 
                 # Tackle the simplest scenario - #members >= #old_assignment
-                if len(members) >= len(old_assignment):
+                if members is not None and len(members) >= len(old_assignment):
                     returnValue(False)
 
                 # Everything else is a temporal state