vulkan/log: Allow but warn if called with a NULL object

vulkan/log: Allow but warn if called with a NULL object

Most of the time when the logging code is invoked, it means we're
already in an edge case.  It should be as robust as possible, otherwise
we risk making hard to debug things even harder.  To that end, instead
of blowing up if passed a NULL object on the list, handle it as
gracefully as we can.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16107>