Don't test is spam_checker not None

Sometimes it's a Mock object which is not none but is still not
what we're after

synapse/events/spamcheck.py

@@ -18,8 +18,14 @@ class SpamChecker(object):
     def __init__(self, hs):
         self.spam_checker = None
 
-        if hs.config.spam_checker is not None:
+        module = None
+        config = None
+        try:
             module, config = hs.config.spam_checker
+        except:
+            pass
+
+        if module is not None:
             self.spam_checker = module(config=config)
 
     def check_event_for_spam(self, event):