[nostalgia/core] Give preload version of AssetRef a default constructor

src/nostalgia/core/assetmanager.hpp

@@ -265,26 +265,28 @@ class AssetManager {
 template<typename T>
 class AssetRef {
 	private:
-		T &m_obj;
+		const T *m_obj = nullptr;
 
 	public:
-		explicit constexpr AssetRef(T &obj) noexcept: m_obj(obj) {
+		constexpr AssetRef() noexcept = default;
+
+		explicit constexpr AssetRef(const T *obj) noexcept: m_obj(obj) {
 		}
 
 		constexpr const T *get() const noexcept {
-			return &m_obj;
+			return m_obj;
 		}
 
 		constexpr const T &operator*() const & noexcept {
-			return &m_obj;
+			return *m_obj;
 		}
 
 		constexpr const T &&operator*() const && noexcept {
-			return &m_obj;
+			return *m_obj;
 		}
 
 		constexpr const T *operator->() const noexcept {
-			return &m_obj;
+			return m_obj;
 		}
 
 		explicit constexpr operator bool() const noexcept {