zink: clear null image surfaces to 0

zink: clear null image surfaces to 0

GL Spec says that imageLoad from incomplete images must return 0.
This is not really spec compliant as for proper behavior nullDescriptor
and robustImageAccess2 is needed.
A workaround for lack of either of these requires a shader variant.
Clearing the null surface and hoping the app doesn't write to the image
is closer to spec, while avoiding a shader recompile.

KHR-GL46.shader_image_load_store.incomplete_textures tests this.

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21135>
(cherry picked from commit 22e91af1a77361249b9c71ee609b67ec187e612c)