Deprecated array_key_exists() on objects detected @ badges/mybadges.php

Detected while testing MDLQA-15375, with PHP 7.4.

The badges/mybadges.php page, throws a good number of PHP deprecation notices:

Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset()

This is about fix them and, ideally, look if there are similar/invalid array_key_exists() uses with objects.

Haven't checked if this also happens with older Moodle versions, may be it's old issue reproducible there, please check.

Ciao