When a script specifies the shell option "nounset" as part of the
shebang (e.g., via "#!/usr/bin/env -S zsh -u"), our initialization
scripts would produce error messages of the form:
__ETC_FOO_SOURCED: parameter not set
These messages could probably be confusing to users when running such
scripts. By providing a fall-back in the parameter expansion, we can
avoid them.
This patch does not address interactive shell start-up, where such
messages may (or may not) be less problematic.
NixOS/nixpkgs@7d84dbdf5b
6ad463a764