var: avoid a segmentation fault when `HOME` is unset

var: avoid a segmentation fault when `HOME` is unset

The code introduced in 576a37fccbf (var: add attributes files locations,
2023-06-27) paid careful attention to use `xstrdup()` for pointers known
never to be `NULL`, and `xstrdup_or_null()` otherwise.

One spot was missed, though: `git_attr_global_file()` can return `NULL`,
when the `HOME` variable is not set (and neither `XDG_CONFIG_HOME`), a
scenario not too uncommon in certain server scenarios.

Fix this, and add a test case to avoid future regressions.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>