Testing the current coreutils git master:

On a Debian 12 system, in which I have not installed a French UTF-8 locale,
I see a test failure of tests/misc/join-utf8.

The essential lines from test-suite.log:

+ test set = set
+ LC_ALL=none
../tests/misc/join-utf8.sh: line 24: warning: setlocale: LC_ALL: cannot change locale (none): No such file or directory

The cause is that on such a system, LOCALE_FR_UTF8, as determined by
gnulib/m4/locale-fr.m4, is 'none', not empty or absent.

The attached patch fixes the failure.