mesa/st: accept NULL and empty buffer objects

mesa/st: accept NULL and empty buffer objects

It's prefectly legal and well-defined to render using a non-existing
or empty buffer object. The data coming out of the buffer object isn't
well defined unless we have the robustness flag set on the context, but
that's a different matter, and up to the shader hardware; it's the same
as out-of-bounds reads.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>