SharedPtr: Add a nullptr constructor

luna/include/luna/SharedPtr.h

@@ -30,6 +30,12 @@ template <typename T> class SharedPtr
     using RefCount = __detail::RefCount;
 
   public:
+    SharedPtr()
+    {
+        m_ptr = nullptr;
+        m_ref_count = nullptr;
+    }
+
     SharedPtr(T* ptr, RefCount* ref_count) : m_ptr(ptr), m_ref_count(ref_count)
     {
     }