Condition not became true on getting values for empty list of keys

Call of `$memd->get([], ...)` doesn't set the condition variables to true. The reason of this is that in `_get()` method `my $servers = $self->{hash}->servers($keys, for => 'get');` returns empty list of servers so request to server doesn't send and callback doesn't call. But `begin()` method for condition variables already called and `end()` method never be call.
