move guard out of is_threepid_reserved and into register.py

changelog.d/4435.bugfix

@@ -1 +1 @@
-Fix None guard in config.server.is_threepid_reserved
+Fix None guard in calling config.server.is_threepid_reserved

synapse/config/server.py

@@ -429,8 +429,6 @@ def is_threepid_reserved(reserved_threepids, threepid):
     Returns:
         boolean Is the threepid undertest reserved_user
     """
-    if not threepid:
-        return False
 
     for tp in reserved_threepids:
         if (threepid['medium'] == tp['medium'] and threepid['address'] == tp['address']):

synapse/rest/client/v2_alpha/register.py

@@ -416,10 +416,11 @@ class RegisterRestServlet(RestServlet):
             )
             # Necessary due to auth checks prior to the threepid being
             # written to the db
-            if is_threepid_reserved(
-                self.hs.config.mau_limits_reserved_threepids, threepid
-            ):
-                yield self.store.upsert_monthly_active_user(registered_user_id)
+            if threepid:
+                if is_threepid_reserved(
+                    self.hs.config.mau_limits_reserved_threepids, threepid
+                ):
+                    yield self.store.upsert_monthly_active_user(registered_user_id)
 
             # remember that we've now registered that user account, and with
             #  what user ID (since the user may not have specified)