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

@@ -416,6 +416,7 @@ class RegisterRestServlet(RestServlet):
             )
             # Necessary due to auth checks prior to the threepid being
             # written to the db
+            if threepid:
                 if is_threepid_reserved(
                     self.hs.config.mau_limits_reserved_threepids, threepid
                 ):