[Vulkan] Crash if vulkan render plugin is preloaded

[Meetem]

It seems like this line in Hook_vkGetInstanceProcAddr should return vkGetInstanceProcAddr(device, funcName); instead of NULL Because will NULL return editor is crashing on start, and no initialization is done.

https://github.com/Unity-Technologies/NativeRenderingPlugin/blob/d6d6f993e7b98731745a7ea5482e46fde01a5ce3/PluginSource/source/RenderAPI_Vulkan.cpp#L115

[Meetem]

Also in example plugin it seems like RenderAPI_Vulkan_OnPluginLoad is never called.

[KsGin]

this patch will fix it #23