Add geom::gradient default constructor

libs/geom/include/psemek/geom/gradient.hpp

@@ -14,6 +14,8 @@ namespace psemek::geom
 	{
 		using result_type = R;
 
+		gradient() = default;
+
 		template <typename ... Args>
 		gradient(Args const & ...  args)
 		{
@@ -29,6 +31,8 @@ namespace psemek::geom
 
 		R operator()(T const & t) const
 		{
+			assert(!points_.empty());
+
 			auto it = std::upper_bound(points_.begin(), points_.end(), t, [](T const & t, auto const & p){ return t < p.first; });
 			if (it == points_.begin())
 				return points_.front().second;