Funny test flake failure: t7518-ident-corner-cases.sh, test 1

Hi,

On my update to my test-selection series, where I simply added more
text to t/README, I saw a failure in the GitHub Actions testing
despite the same series having previously passed without that extra
text.  The extra text was unrelated; the failure is due to unsafe
environment assumptions (though ones I probably would have made
too...).  See the actual output here:

https://github.com/git/git/pull/878/checks?check_run_id=1266009463

The issue comes from this test code:

       (
               sane_unset GIT_AUTHOR_EMAIL &&
               GIT_AUTHOR_NAME= &&
               test_must_fail git commit --allow-empty -m foo 2>err &&
               test_i18ngrep ! null err
       )

The last line expects 'null' to NOT be found in err, unfortunately,
the file err contained the line:

fatal: empty ident name (for
[off-list ref])
not allowed
218

Note that 'null' appears as a substring of the domain name, found
within 'gcliasfzo2nullsdbrimjtbyhg'.


This testcase was introduced in commit 862e80a413 ("ident: handle NULL
email when complaining of empty name", 2017-02-23), and the reason for
the check appears in a comment above the test:

+# confirm that we do not segfault _and_ that we do not say "(null)", as
+# glibc systems will quietly handle our NULL pointer

Should we tighten the test to check for "(null)" instead of "null", or
should we do something else?  Or just ignore it as it is somewhat
unlikely that anyone ever hits this flake again?