message: Fix construction of zero-sized arrays of `DynamicArrayReference`
  Even though there are no elements in the array, the element type must have a valid default constructor.
diff: @@ -34,7 +34,7 @@ struct DynamicArrayReference : ArrayBase<value_t<Packet_>, DynamicArrayReference
  static constexpr size_t PacketSize = Packet::Size;
  static constexpr bool IsMask = Packet::IsMask;
  - DynamicArrayReference(Packet *packets) : m_packets(packets) { }
  + DynamicArrayReference(Packet *packets = nullptr) : m_packets(packets) { }
  ENOKI_INLINE Packet &packet(size_t i) {
  return ((Packet *) ENOKI_ASSUME_ALIGNED(m_packets, alignof(Packet)))[i];
  