log pdu_failures from incoming transactions

... even if we have no EDUs.

This appears to have been introduced in
476899295f.

synapse/federation/federation_server.py

@@ -168,7 +168,8 @@ class FederationServer(FederationBase):
                     edu.content
                 )
 
-            for failure in getattr(transaction, "pdu_failures", []):
+        pdu_failures = getattr(transaction, "pdu_failures", [])
+        for failure in pdu_failures:
             logger.info("Got failure %r", failure)
 
         response = {