Check that XDG base directories paths are absolute

The [spec][1] reads:

> All paths set in these environment variables must be absolute. If an
> implementation encounters a relative path in any of these variables it should
> consider the path invalid and ignore it.

and

> If $XDG_DATA_HOME is either not set or empty, a default equal to
> $HOME/.local/share should be used.

Testing that the path is absolute also entails that is is non-empty.

[1]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html



Signed-off-by: Antonin Décimo <antonin.decimo@gmail.com>