Add magic __isset to classes with __get

[MGatner]

The following classes have a magic __get() method without a corresponding __isset():

• CodeIgniter4/system/Database/BaseConnection.php
• CodeIgniter4/system/Model.php
• CodeIgniter4/system/Session/Session.php
• CodeIgniter4/system/I18n/Time.php
• CodeIgniter4/system/CLI/BaseCommand.php
• CodeIgniter4/system/I18n/TimeDifference.php
• CodeIgniter4/system/Encryption/Encryption.php
• CodeIgniter4/system/Encryption/Handlers/BaseHandler.php

In general this is a bad idea because some functions that call language constructs like isset() on an instance will fail to return true when __get() would still return a valid item. See #2205 for one example.

Unless there's some compelling reason not to, I propose each of these classes should have a basic __isset() added, modeled after their __get(). E.g.:

    public function __isset($name)
    {
        return isset($this->attributes[$name]);
    }

[lonnieezell]

No compelling reason not to do that.

[jim-parry]

Fixed by #2231