Check empty should be a field configuration, not a plugin annotation

@Luke.Leber - I'm thinking the plugin annotation was just to determine whether the check_empty could be set for that particular field type. What might be wrong is the settings for each field should be driving the isEmpty() evaluation vs. the definition. See https://git.drupalcode.org/project/custom_field/-/blob/1.0.x/src/Plugin/Field/FieldType/CustomItem.php#L741

Ah, I think that I see.

Yeah, the root of what I observed while debugging was that https://git.drupalcode.org/project/custom_field/-/blob/1.0.x/src/Plugin/... evaluates FALSE even when one checks the 'Check empty?' checkbox on the field settings.

I was able to add an interface method and plumb it in on #L748 to get things working as expected. Maybe we need to check if the plugin definition says to check empty or the field settings say to check empty, then do the check?

Maybe as a follow-up we should set the default empty check value in the field settings based on the plugin annotation value (unless I'm misinterpreting the intent, of course!)?

Yeah, the annotation doesn't get involved with the value. It's just to determine if the field widget is eligible for the setting. uuid (value is auto set) & checkbox (0 value would always be empty) are the 2 i can think of that relevant.

This patch should fail testing to prove there's something here to fix.

...and this should fix it!

Work left to do is to clean up the test and see if we can use a unified test data generator. The arrays are pretty unwieldy to maintain manually.

@Luke.Leber - patch is great. Thanks! Merged into dev branch.