api: set event_types to empty list if not supplied in register.

Previously, if you didn't specify a list of `event_types` in either `register` or `call_on_each_event`, you'd get an exception due to the invalid event format.

This fixes #86.

zulip/zulip/__init__.py

@@ -616,6 +616,9 @@ class Client(object):
         if narrow is None:
             narrow = []
 
+        if event_types is None:
+            event_types = []
+
         request = dict(
             event_types=event_types,
             narrow=narrow,