[5.x] Fix emptiness check on Value properties

[godismyjudge95]

Tricky behavior ran into by Erin - https://discord.com/channels/489818810157891584/489830535565148169/1333967156861468694

This won't directly solve what he was dealing with (as he had a custom class), but it will fix it for future devs checking if a property on a Value is empty.

Currently, if you have an object wrapped by a Value and then check the emptiness of a property on that object it returns true no matter what:

This is due to https://www.php.net/manual/en/function.empty.php#99959

Calling non existing object property, empty($object->prop), will trigger __isset(), the same way as isset($object->prop) does, but there is one difference. If __isset() returns TRUE, another call to __get() will be made and actual return value will be result of empty() and result of __get().

In other words, since Value doesn't have the __isset() method on it every single empty() check on a property inside of it will fail.

This PR fixes that and adds a test :)