Fix logic for default XDG_DATA_DIRS value

[iFreilicht]

Motivation

Fixes an oversight in #8985.

Context

The POSIX test manpage as well as the fish test manpage specify that -z will be "True if the length of string string is zero; otherwise, false."

The -n was likely a mixup and not caught during testing of #8985 due to a lack of missing conflicting entries in XDG_DATA_DIRS.

Priorities

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[iFreilicht]

I already tested this change and can confirm that now, after installing the version of Nix from this branch and starting a new terminal session, XDG_DATA_DIRS is set correctly:

$ echo $XDG_DATA_DIRS
/home/felix/.local/share/flatpak/exports/share:/var/lib/flatpak/exports/share:/usr/local/share:/usr/share:/var/lib/snapd/desktop:/home/felix/.nix-profile/share:/nix/var/nix/profiles/default/share

@Hoverbear can you confirm that after applying the same testing from your original PR, you get the expected results?

[Hoverbear]

Oh no. 😓 It seems I wasn't thinking and copied from the NIX_SSL_CERT_FILE lines below. (#8985 (comment))

This makes logical sense and I can confirm on my test VM this resolves the issue described.

Here is the result from #8985:

[Ericson2314]

I don't really know how the installer tests work, but it would be nice to test this. Can one of you two take a stab at that?

[Hoverbear]

I believe this fixes the problem.

[Hoverbear]

A test could be added somewhere like

nix/tests/installer/default.nix

Lines 210 to 219 in fe62cb7

	source ~/.bash_profile || true
	source ~/.bash_login || true
	source ~/.profile || true
	source /etc/bashrc || true
	nix-env --version
	nix --extra-experimental-features nix-command store info
	out=\$(nix-build --no-substitute -E 'derivation { name = "foo"; system = "x86_64-linux"; builder = "/bin/sh"; args = ["-c" "echo foobar > \$out"]; }')
	[[ \$(cat \$out) = foobar ]]

.

I believe merging this bugfix independently of an added test would cause the existing bug to impact less people.

[workingjubilee]

Maybe we can call it good if we open an issue for the regression test so that it's being tracked?

[fricklerhandwerk]

Maybe we can call it good if we open an issue for the regression test so that it's being tracked?

@Hoverbear please do!