Update null_window.cpp

Add missing swCreateWindow function for null impl

src/platform/null_window.cpp

@@ -17,3 +17,8 @@ bool ShouldClose(const NullWindowImpl&)
 }
 
 } // namespace starter_window
+
+starter_window::Window swCreateWindow(starter_window::WindowCreateParams)
+{
+	return starter_window::NullWindowImpl{};
+}