The '[' and 'test' implementations from GNU coreutils don't support '-v'
as a way to check if a shell variable is set [1]. Only Bash's built-in
implementations do.
This is quite confusing and makes it difficult to find out what '-v'
actually does. eg., 'man --all test' only shows the manual for the GNU
coreutils version, which doesn't list '-v' [1], and, 'man --all [' only
shows the manual for Bash's built-ins, which also doesn't list '-v'.
One has to go to the bash(1) manual to find it [2].
Elsewhere in the code base [3], the same thing is accomplished with '-z'
and parameter substitution, which are more widely supported and, hence,
easier to find documentation for.
[1] https://manpages.debian.org/testing/coreutils/test.1.en.html
[2] https://linux.die.net/man/1/bash
[3] Commit 84ae385f33https://github.com/containers/toolbox/pull/1334https://github.com/containers/toolbox/pull/1341
341ae55f9d