Explicitly handle undefined case for getting web_dispatch.auth_backends

(cherry picked from commit b619e66)

Author: aaron-seo

deps/rabbitmq_web_dispatch/src/rabbit_web_dispatch_access_control.erl

@@ -366,7 +366,7 @@ is_basic_auth_disabled(#auth_settings{basic_auth_enabled = Enabled}) ->
 get_auth_backends() ->
     case application:get_env(rabbitmq_web_dispatch, auth_backends) of
 		{ok, Backends} -> {ok, Backends};
-		_ -> rabbit_log:debug("rabbitmq_web_dispatch.auth_backends not configured,
+		undefined -> rabbit_log:debug("rabbitmq_web_dispatch.auth_backends not configured,
                                 falling back to rabbit.auth_backends"),
-             application:get_env(rabbit, auth_backends)
+                     application:get_env(rabbit, auth_backends)
 	end.