nixos-install: Fix chroot flag not defaulting to Bash outside NixOS.

Passing the chroot flag to nixos-install without arguments should now give you a
Bash shell as intended rather than try an empty path.

This was masked by the user's shell (usually /bin/bash) being defaulted to by
chroot, and being found since their paths used NixOS conventions.

nixos/modules/installer/tools/nixos-install.sh

@@ -45,7 +45,9 @@ while [ "$#" -gt 0 ]; do
             ;;
         --chroot)
             runChroot=1
-            chrootCommand=("$@")
+            if [[ "$@" != "" ]]; then
+                chrootCommand=("$@")
+            fi
             break
             ;;
         --help)