message: Fix MSVC runtime failure for undef std::nullptr_t
diff: @@ -800,10 +800,18 @@ 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;
  return undef;
  }
  }
  +}
  +
  +#if defined(_MSC_VER)
  +# pragma warning(pop)
  +#endif
  /// Create a dummy memory region that can be used to capture computation symbolically
  template <typename T>
  @@ -824,10 +832,6 @@ ENOKI_INLINE decltype(auto) placeholder(const T &value, bool preserve_size, bool
  }
  }
  -#if defined(_MSC_VER)
  -# pragma warning(pop)
  -#endif
  -
  template <typename T, typename T2>
  ENOKI_INLINE T full(const T2 &value, size_t size = 1) {
  ENOKI_MARK_USED(size);
  