Rework Dictionary::TryGet to use std::optional.

src/Collections/Dictionary.hpp

@@ -46,13 +46,12 @@ namespace ArbUt {
 
         /// @brief Try to get an item from the dictionary using a key. Returns false if no item is found, and out will
         /// not be touched in that case.
-        inline bool TryGet(const KeyT& key, ValueT& out) const noexcept {
+        inline std::optional<std::reference_wrapper<const ValueT>> TryGet(const KeyT& key) const noexcept {
             const auto& find = _map.find(key);
             if (find == _map.end()) {
-                return false;
+                return {};
             }
-            out = find->second;
-            return true;
+            return std::ref(find->second);
         }
 
         /// @brief Removes an item with a certain key from the dictionary