dom0-updates: display GUI notification about no updates

If no updates are available, display some notification about that (when
GUI mode requested). Otherwise user has no idea what happened (update
failed? still doing something?).

Fixes QubesOS/qubes-issues#3751

dom0-updates/qubes-dom0-update

@@ -229,4 +229,7 @@ elif [ -f /var/lib/qubes/updates/repodata/repomd.xml ]; then
 else
     rm -f $UPDATES_STAT_FILE
     echo "No updates available" >&2
+    if [ "$GUI" == "1" ]; then
+        zenity --title='Dom0 updates' --text='No updates available'
+    fi
 fi