Allow running without arguments

https://github.com/alexbelgium/hassio-addons/issues/565#issuecomment-1557319726

portainer_agent/rootfs/etc/services.d/portainer_agent/run

@@ -11,4 +11,8 @@ bashio::log.info "Starting app"
 
 # Launch app
 cd /app || true
-./agent "$PORTAINER_AGENT_ARGS" 2>/dev/null || ./agent
+if bashio::config.has_value 'PORTAINER_AGENT_ARGS'; then
+  ./agent "$PORTAINER_AGENT_ARGS" 2>/dev/null
+else
+  ./agent
+fi