radv: fix crash in vkCmdClearAttachments with unused attachment

depth_stencil_attachment and/or ds_resolve attachment can be NULL.

This fixes crashes with
dEQP-VK.renderpass.suballocation.unused_clear_attachments.*

Cc: 19.1 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit b5116d3c)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/amd/vulkan/radv_meta_clear.c