FIX Handle getting HasOneRelationFieldInterface passed as an array

[emteknetnz]

Issue https://github.com/silverstripe/silverstripe-elemental/issues/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]

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

[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