[5.2] mod_submenu php fixes

[peter1szalatnay]

Summary of Changes

Fix an issues where xdebug always stop because asset is always missing from list, and it is just better then having @ for error handling.

Testing Instructions

Enable xdebug and go to the system page.

Actual result BEFORE applying this Pull Request

Exception has occurred. Warning: Undefined array key 1

Expected result AFTER applying this Pull Request

No errors

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [x] No documentation changes for docs.joomla.org needed

• [ ] Pull Request link for manual.joomla.org:

• [x] No documentation changes for manual.joomla.org needed