Return nil if service doesn't respond to any handling method

Without the explicit nil, the list of attempted methods is returned and calling
code may incorrectly interpret this as a valid result and attempt to use it,
causing confusing error messages.

Author: pbrisbin

lib/cc/service.rb

@@ -86,6 +86,8 @@ def receive
           return public_send(method)
         end
       end
+
+      nil
     end
 
     private