Not compiling when VMA_EXTERNAL_MEMORY_WIN32 disabled.

[sagaceilo]

https://github.com/GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator/blob/de8e65796ae5bf780a828c47376e6744bf1f4336/include/vk_mem_alloc.h#L10597

Hey,. this line is not compiling, when define switch is not specified. In one scenario VmaWin32Handle is a class with explicite handle construcor, but in other it has no public constructor at all.

Solution : remove VMA_NULL and leave only m_Handle() or remove line completly as it will fallback to default constructor in both cases.

Thanks :)