vk_buffer_cache: Add support for null index buffers

Games can bind a null index buffer (size=0) where all indices are
evaluated as zero. VK_EXT_robustness2 doesn't support this and all
drivers segfault when a null index buffer is passed to
vkCmdBindIndexBuffer.

Workaround this by creating a 4 byte buffer and filling it with zeroes.
If it's read out of bounds, robustness takes care of returning zeroes as
indices.