Add a constructor for PtrUnion in case it is not initialized explicitly (#1064)

[ROCm/rccl commit: ba9f7917ba]

projects/rccl/test/common/PtrUnion.hpp

@@ -47,6 +47,8 @@ namespace RcclUnitTesting
     double*        F8; // ncclFloat64
     rccl_bfloat16* B2; // ncclBfloat16
 
+    constexpr PtrUnion() : ptr(nullptr) {}
+
     ErrCode Attach(void *ptr);
     ErrCode Attach(PtrUnion ptrUnion);