Cocoa: Fix glfwGetCocoaWindow error return value

src/cocoa_window.m

@@ -2047,7 +2047,7 @@ GLFWAPI id glfwGetCocoaWindow(GLFWwindow* handle)
     {
         _glfwInputError(GLFW_PLATFORM_UNAVAILABLE,
                         "Cocoa: Platform not initialized");
-        return NULL;
+        return nil;
     }
 
     return window->ns.object;