layers: Special case threading and VK_NULL_HANDLE
layers: Special case threading and VK_NULL_HANDLE

If a parameter is VK_NULL_HANDLE, then it needs no use count.

Fixes #1712

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers