Add explicit return types to `ipl-orm`-derived methods to prepare for strict
typing. These additions are safe, as they only annotate existing methods that
previously lacked return type declarations.
Passing `null` as the key to `array_key_exists()` or using it as an array key
is deprecated as of PHP 8.5.
Co-authored-by: Eric Lippmann <eric.lippmann@icinga.com>