fix known_users function signature

refs AE-92

Change-Id: I4d31a35831d62fb423228e38956bdd04f39a1195
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/307327
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jacob Burroughs <jburroughs@instructure.com>
QA-Review: Aaron Ogata <aogata@instructure.com>
Product-Review: Aaron Ogata <aogata@instructure.com>

lib/address_book/empty.rb

@@ -23,7 +23,7 @@ module AddressBook
   # shapes but no data -- while being separate from the original
   # MessageableUser strategy.
   class Empty < AddressBook::Base
-    def known_users(_users, **)
+    def known_users(_users, *)
       []
     end