message: Fix MSVC Runtime error checks with undef variables
diff: @@ -800,14 +800,14 @@ template <typename T> ENOKI_INLINE T empty(size_t size = 1) {
  "Unsupported data structure -- did you forget to include "
  "'enoki/struct.h' or provide a suitable ENOKI_STRUCT() "
  "declaration?");
  - if constexpr (std::is_same_v<T, std::nullptr_t>) {
  - return nullptr;
  - } else {
  +
  T undef;
  + #if defined(_MSC_VER) && !defined(NDEBUG) // MSVC's /RTC (Runtime error checks) requires the following
  + T* undef_ptr = &undef;
  + #endif
  return undef;
  }
  }
  -}
  #if defined(_MSC_VER)
  # pragma warning(pop)
  