nix print-dev-env assumes that PATH and XDG_DATA_DIRS actually exist

[jfly]

I use direnv with strict_env enabled, and with this Hand-rolled Nix flakes integration

This demo.sh script effectively shows off what direnv does when I cd into a flake-enabled project:

$ cat demo.sh
#!/usr/bin/env bash

set -euo pipefail
eval "$(nix print-dev-env)"

This recently started failing for me after #6702. Here's what started happening:

$ ./demo
./demo.sh: line 6: XDG_DATA_DIRS: unbound variable

Note how nix print-dev-env now assumes that XDG_DATA_DIRS exists:

$ nix print-dev-env
unset shellHook
nix_saved_PATH="$PATH"
nix_saved_XDG_DATA_DIRS="$XDG_DATA_DIRS"
...

(There was already the same problem with PATH, but I've always had that
environment variable defined, I just don't have a XDG_DATA_DIRS set to
anything.)

I'll put together a PR to not assume that these environment variables are
actually set.