Fix util::ndarray::copy() in case of an empty array

libs/util/include/psemek/util/ndarray.hpp

@@ -379,6 +379,9 @@ namespace psemek::util
 	template <typename T, std::size_t N>
 	ndarray<T, N> ndarray<T, N>::copy() const
 	{
+		if (empty())
+			return {};
+
 		std::unique_ptr<T[]> data(new T[size()]);
 		std::copy(begin(), end(), data.get());
 		return ndarray{dims_, std::move(data)};