Add comment and simplify diff

synapse/config/server.py

@@ -111,11 +111,11 @@ class ServerConfig(Config):
         self.admin_contact = config.get("admin_contact", None)
 
         # FIXME: federation_domain_whitelist needs sytests
+        self.federation_domain_whitelist = None
         federation_domain_whitelist = config.get(
             "federation_domain_whitelist", None,
         )
 
-        self.federation_domain_whitelist = None
         if federation_domain_whitelist is not None:
             # turn the whitelist into a hash for speed of lookup
             self.federation_domain_whitelist = {}

synapse/config/tls.py

@@ -98,6 +98,9 @@ class TlsConfig(Config):
         self.federation_ca_trust_root = None
         if custom_ca_list is not None:
             if len(custom_ca_list) == 0:
+                # A trustroot cannot be generated without any CA certificates.
+                # Raise an error if this option has been specified without any
+                # corresponding certificates.
                 raise ConfigError("federation_custom_ca_list specified without "
                                   "any certificate files")