fix(pve-post-install): check file exists before removing no-nag-script

Fixes script interruption when selecting 'no' to disable subscription nag
on a fresh system where no-nag-script doesn't exist.

tools/pve/post-pve-install.sh

@@ -595,7 +595,7 @@ EOF
   no)
     whiptail --backtitle "Proxmox VE Helper Scripts" --msgbox --title "Support Subscriptions" "Supporting the software's development team is essential. Check their official website's Support Subscriptions for pricing. Without their dedicated work, we wouldn't have this exceptional software." 10 58
     msg_error "Selected no to Disabling subscription nag"
-    rm /etc/apt/apt.conf.d/no-nag-script 2>/dev/null
+    [[ -f /etc/apt/apt.conf.d/no-nag-script ]] && rm /etc/apt/apt.conf.d/no-nag-script
     ;;
   esac
   apt --reinstall install proxmox-widget-toolkit &>/dev/null || msg_error "Widget toolkit reinstall failed"