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

src/video_core/renderer_vulkan/vk_buffer_cache.cpp

@@ -176,7 +176,7 @@ void BufferCacheRuntime::BindVertexBuffer(u32 index, VkBuffer buffer, u32 offset
                                           u32 stride) {
     if (device.IsExtExtendedDynamicStateSupported()) {
         scheduler.Record([index, buffer, offset, size, stride](vk::CommandBuffer cmdbuf) {
-            const VkDeviceSize vk_offset = offset;
+            const VkDeviceSize vk_offset = buffer != VK_NULL_HANDLE ? offset : 0;
             const VkDeviceSize vk_size = buffer != VK_NULL_HANDLE ? size : VK_WHOLE_SIZE;
             const VkDeviceSize vk_stride = stride;
             cmdbuf.BindVertexBuffers2EXT(index, 1, &buffer, &vk_offset, &vk_size, &vk_stride);