fish: verify if systemctl exists before trying to use it

fish/.config/fish/config.fish

@@ -31,7 +31,7 @@ set -x PAGER less
         exec startx i3
     end
     if test "$XDG_VTNR" = 1 -a -z "$DISPLAY" #faster like this
-        if systemctl -q is-active graphical.target 
+        if command -v systemctl -a systemctl -q is-active graphical.target 
             export _JAVA_AWT_WM_NONREPARENTING=1
             export GTK_CSD=0
             export LD_PRELOAD=/usr/lib/libgtk3-nocsd.so.0