Re: [PATCH] var: avoid a segmentation fault when `HOME` is unset

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> 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.