nix-profile{,-daemon}.sh.in: Allow XDG_STATE_HOME to be unset

One of our CI machines installs Nix via the official script and then
sources the nix-profile.sh script to setup the environment. However, it
doesn't have XDG_STATE_HOME set, which causes sourcing the script to
fail.

(cherry picked from commit 24eaa08)

scripts/nix-profile-daemon.sh.in

@@ -3,7 +3,7 @@ if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
 __ETC_PROFILE_NIX_SOURCED=1
 
 NIX_LINK=$HOME/.nix-profile
-if [ -n "$XDG_STATE_HOME" ]; then
+if [ -n "${XDG_STATE_HOME-}" ]; then
     NIX_LINK_NEW="$XDG_STATE_HOME/nix/profile"
 else
     NIX_LINK_NEW=$HOME/.local/state/nix/profile

scripts/nix-profile.sh.in

@@ -3,7 +3,7 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
     # Set up the per-user profile.
 
     NIX_LINK="$HOME/.nix-profile"
-    if [ -n "$XDG_STATE_HOME" ]; then
+    if [ -n "${XDG_STATE_HOME-}" ]; then
         NIX_LINK_NEW="$XDG_STATE_HOME/nix/profile"
     else
         NIX_LINK_NEW="$HOME/.local/state/nix/profile"