Fix build issue on older compilers

The older compilers used in CI cannot deduce the type of the argument to std::optional::value_or()

Author: cpp-niel

src/node/hlist.cpp

@@ -23,7 +23,7 @@ namespace mfl
     {
         return std::ranges::fold_left_first(
                    l.nodes | std::views::transform([](const node_variant& n) { return width(n); }), std::plus{})
-            .value_or({});
+            .value_or(dist_t{});
     }
 
     dist_t hlist_depth(const hlist& l)

src/node/vlist.cpp

@@ -9,6 +9,6 @@ namespace mfl
     {
         return std::ranges::fold_left_first(
                    l.nodes | std::views::transform([](const node_variant& n) { return vsize(n); }), std::plus{})
-            .value_or({});
+            .value_or(dist_t{});
     }
 }