radv: rely on non-NULL binaries when inserting shaders to the cache

radv: rely on non-NULL binaries when inserting shaders to the cache

With GPL, a stage can be imported from a library which means that the
binary is NULL (it's freed right after compilation) but the shader is
non-NULL. To avoid crashing, rely on non-NULL binaries because this
implies that the shader is non-NULL as well.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22264>