FIX Handle getting HasOneRelationFieldInterface passed as an array

[emteknetnz]

Issue silverstripe/silverstripe-elemental#1155

This fixes bugs with autoscaffolded SearchableDropdownField for has_one SiteTree relations

Replication steps

• Create a non-inline editable elemental content block
• Add a has_one relation to a 'MySiteTree' => SiteTree::class
• Add include the validation php block below

    public function getCMSCompositeValidator(): CompositeValidator
    {
        return CompositeValidator::create([RequiredFields::create(['MySiteTreeID'])]);
    }

You can leave the field blank and submit and the required fields validation won't trigger

[GuySartorelli]

I think the validator is working correctly - the developer should use the correct field name when setting their validator, i.e. it should be

RequiredFields::create(['MySiteTreeID'])

and not

RequiredFields::create(['MySiteTree'])

[GuySartorelli]

I'll treat this PR as an enhancement instead of a bugfix, since I think the current behaviour isn't buggy, even though it's potentially confusing since some fields use the ID suffix and some don't.

To that end, please update the commit prefix to ENH.

[GuySartorelli]

Does the bug you're fixing affect CMS 5.2?
If so, please retarget to the 5.2 branch since it's fixing a bug

[GuySartorelli]

I've tried to reproduce the bug mentioned in the description both from the 5 and 5.2 branch - I can't reproduce it from either. Validation works fine without this PR in both scenarios.

Please try spinning up a fresh installation and testing your own reproduction steps.

[emteknetnz]

Yeah looks like we don't need this one, I think there may have been some initial confusion with the "ID" suffix making me think things were broken when they were not

[emteknetnz]

Turns out we do need this one, it's required for doing a page save of an inline-block where the has_one SiteTree::class in in RequiredFields. It's NOT required for saving non-inline block's.

It showed up as a behat failure in page-save-validation.feature on https://github.com/silverstripe/silverstripe-elemental/actions/runs/8961905824/job/24613378755?pr=1178#step:12:1052

[GuySartorelli]

It turns out there's a difference with some fields between "I have not selected anything" and "I have specifically clicked on the dropdown and selected the empty option".

Since form submission already correctly handles both cases, and it's been like that for donkey's years, I think it makes sense to handle this here as you've done.