vk_buffer_cache: Fix offset for NULL vertex buffers

The Vulkan spec states:
If an element of pBuffers is VK_NULL_HANDLE, then the corresponding element of pOffsets must be zero.

https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/vkCmdBindVertexBuffers2EXT.html#VUID-vkCmdBindVertexBuffers2EXT-pBuffers-04112