Label containers with empty destinations

This will allow us to filter for containers that have no destination in
cases where we deploy an empty + a non empty destination to the same
host.

To note:

```
\# Containers with a destination label
$ docker ps --filter label=destination

\# Containers with an empty destination label
$ docker ps --filter label=destination=
```

lib/kamal/configuration/role.rb

@@ -176,11 +176,7 @@ class Kamal::Configuration::Role
     end
 
     def default_labels
-      if config.destination
-        { "service" => config.service, "role" => name, "destination" => config.destination }
-      else
-        { "service" => config.service, "role" => name }
-      end
+      { "service" => config.service, "role" => name, "destination" => config.destination }
     end
 
     def traefik_labels