SPIRV validation error with assign of nullptr to pointer variable

[dubiousconst282]

Minimal repro:

[vk::push_constant] int* dest;

[numthreads(1)]
void ComputeMain(int tid : SV_DispatchThreadID) {
    int* ptr = nullptr;
    if (tid % 2 == 0) ptr = dest; // prevent const fold
    if (ptr) *ptr = 123;
}

Produces:

$ slangc -target spirv logs/shader-test/main.slang | spirv-val --target-env vulkan1.3
error: line 56: OpStore Pointer <id> '8[%ptr]'s type does not match Object <id> '28[%28]'s type.
  OpStore %ptr %28

The problem is that nullptr is being lowered into a void pointer as OpConvertUToPtr %_ptr_PhysicalStorageBuffer_void %ulong_0 instead of the target pointer type.

A workaround is to write (T*)0 instead of nullptr.