Fix exception if an optional arg was None in the internal API

The arg when_stop of updates.UpdatesFetcher.block_until_alone should be
supposed to also be None, but that resulted in an exception before this
commit.

Author: Pietro Albini

botogram/updates.py

@@ -123,7 +123,7 @@ def block_until_alone(self, treshold=4, check_timeout=1, when_stop=None):
 
         while checks_count < treshold:
             # This provides an artificial end to the blocking
-            if when_stop():
+            if when_stop is not None and when_stop():
                 return False
 
             try: