FIX DBForeignKey scaffolding missing parameter

[johannesx75]

Description

Auto-scaffolding for has_one form fields creates an instance of SearchableDropdownField by passing:
SearchableDropdownField::create($this->name, $title, $list, $labelField)

The signature for the SearchableDropdownField constructor expects:
string $name, ?string $title = null, ?DataList $source = null, mixed $value = null, string $labelField = 'Title'

So the call is missing $value. This leads to $value being set to $labelField, which is 'Title'.

Manual testing steps

Given ModelA has_one ModelB and ModelB has_many ModelA:

• Open ModelB in Model Admin
• Go to the ModelA Tab
• Click on Add new Model A
• Check Error Log.

Prior to this fix it would contain a warning:

Could not determine value in SilverStripe|Forms|SearchableDropdownField: :getValueArray()

Issues

• DBForeignKey scaffolding uses wrong parameters to create SearchableDropdownField #11294

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