[sh] Fix comparison error for autostart x script

When no virtual terminal is set (as an XDG value by e.g. the linux
installation), fixes error complaining about comparing nothing with an
integer.

sh/.config/sh/profile.d/autostartx.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env sh
 
-if [ ! "$DISPLAY" ] && [ "$XDG_VTNR" -eq 1 ]; then
+if [ ! "$DISPLAY" ] && [ -n "$XDG_VTNR" ] && [ "$XDG_VTNR" -eq 1 ]; then
   exec startx "$XDG_CONFIG_HOME"/xresources/xinitrc
 fi