Merge pull request #3754 from matrix-org/erikj/fix_whitelist

Allow federation_domain_whitelist to be emtpy list

changelog.d/3754.bugfix

@@ -0,0 +1 @@
+Fix 'federation_domain_whitelist' such that an empty list correctly blocks all outbound federation traffic

synapse/http/matrixfederationclient.py

@@ -133,7 +133,7 @@ class MatrixFederationHttpClient(object):
                 failures, connection failures, SSL failures.)
         """
         if (
-            self.hs.config.federation_domain_whitelist and
+            self.hs.config.federation_domain_whitelist is not None and
             destination not in self.hs.config.federation_domain_whitelist
         ):
             raise FederationDeniedError(destination)