[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:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Quy]

I have tested this item ✅ successfully on 2d2ddbc

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[viocassel]

I have tested this item ✅ successfully on 2d2ddbc

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/44320.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/44320.}

[Hackwar]

Thanks for your contribution!