Guard against heartbeat task not having started when we're shutdown

It can happen if there's a fundamental error that makes us fail as soon as
we try to start ^_^U (for example, the DB we're trying to connect not
existing).

Author: rosa

lib/solid_queue/process_registration.rb

@@ -54,7 +54,7 @@ def launch_heartbeat
       end
 
       def stop_heartbeat
-        @heartbeat_task.shutdown
+        @heartbeat_task&.shutdown
       end
 
       def heartbeat