Replacing NULL with nullptr in examples/backends when they creeped back. (#6313, #7071, #4537)

examples/example_win32_directx12/main.cpp

@@ -58,7 +58,7 @@ struct ExampleDescriptorHeapAllocator
     }
     void Destroy()
     {
-        Heap = NULL;
+        Heap = nullptr;
         FreeIndices.clear();
     }
     void Alloc(D3D12_CPU_DESCRIPTOR_HANDLE* out_cpu_desc_handle, D3D12_GPU_DESCRIPTOR_HANDLE* out_gpu_desc_handle)