nix-daemon.sh profile script: operate under set -u

If the profile is sourced inside a script with `set -u`, the check for
__ETC_PROFILE_NIX_SOURCED and NIX_SSL_CERT_FILE would raise an error.
A simple guard around this check allows the script to operate under
standard environments (where it is fairly reasonable to assume USER
and HOME are set.)

scripts/nix-profile-daemon.sh.in

@@ -1,5 +1,5 @@
 # Only execute this file once per shell.
-if [ -n "$__ETC_PROFILE_NIX_SOURCED" ]; then return; fi
+if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
 __ETC_PROFILE_NIX_SOURCED=1
 
 # Set up secure multi-user builds: non-root users build through the
@@ -51,7 +51,7 @@ fi
 
 
 # Set $NIX_SSL_CERT_FILE so that Nixpkgs applications like curl work.
-if [ ! -z "$NIX_SSL_CERT_FILE" ]; then
+if [ ! -z "${NIX_SSL_CERT_FILE:-}" ]; then
     : # Allow users to override the NIX_SSL_CERT_FILE
 elif [ -e /etc/ssl/certs/ca-certificates.crt ]; then # NixOS, Ubuntu, Debian, Gentoo, Arch
     export NIX_SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt