make reference tags parsing work with generated fields

[i-just]

Description

Adds the ability to parse generated fields in reference tags.

To test:

• create a section with an entry type that has a generated field with a handle of myTest and a value of abcde
• create an entry in that section
• create a twig template with the following code: <p>{{ craft.app.elements.parseRefs('{entry:194:myTest}') }}</p> (replace the number with the actual ID of the entry from the previous step)
• view that template in a browser

Before this change, the result would be the entry’s URL (if the section is configured with URIs) or an empty string (if configured without URIs).

With this change, you should see abcde.

Related issues

#18692

[brandonkelly]

This ended up being just a bug in Element::__isset(), which wasn’t checking for generated field values, so I fixed it there, and that felt like a reasonable 5.9.x change.