install-grub: only try to loadfont if font is not null

nixos/modules/system/boot/loader/grub/install-grub.pl

@@ -281,6 +281,11 @@ else {
         else
           insmod vbe
         fi
+    ";
+
+    if ($font) {
+        copy $font, "$bootPath/converted-font.pf2" or die "cannot copy $font to $bootPath\n";
+        $conf .= "
             insmod font
             if loadfont " . $grubBoot->path . "/converted-font.pf2; then
               insmod gfxterm
@@ -294,9 +299,6 @@ else {
               terminal_output gfxterm
             fi
         ";
-
-    if ($font) {
-        copy $font, "$bootPath/converted-font.pf2" or die "cannot copy $font to $bootPath\n";
     }
     if ($splashImage) {
         # Keeps the image's extension.