XDG environment variables with relative paths should be ignored

XDG Base Directory Specification (https://specifications.freedesktop.org/basedir-spec/basedir-spec-0.8.html) says about $XDG_* variables:
If an implementation encounters a relative path in any of these variables it should consider the path invalid and ignore it.

But QStandardPaths::writableLocation uses values from XDG variables also when they contain relative paths.