Update lib/AppInfo/Application.php

The current change is currently filling the nextcloud logs up with the error:
`Undefined array key \"HTTP_USER_AGENT\" at /srv/www/nextcloud/apps/side_menu/lib/AppInfo/Application.php#109`

This adds a check to ensure the index exists before read it.

lib/AppInfo/Application.php

@@ -106,7 +106,7 @@ class Application extends App implements IBootstrap
 
     protected function isEnabled(): bool
     {
-        if (preg_match('/MemoriesNative/', $_SERVER['HTTP_USER_AGENT'])) {
+        if (isset($_SERVER["HTTP_USER_AGENT"]) && preg_match('/MemoriesNative/', $_SERVER['HTTP_USER_AGENT'])) {
             return false;
         }