Using PHP7 type hinting with Symfony Form Text Type and empty_data returning NULL

ChangeInspectionDetails.php

<?php

class ChangeInspectionDetails extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('problemNumber', TextType::class, [
                'empty_data' => '' // no effect
            ]);
    }
    

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => ChangeInspectionDetailsCommand::class
        ]);
    }    
}

ChangeInspectionDetailsCommand.php

<?php

class ChangeInspectionDetailsCommand
{
    public $problemNumber = ''; // no effect
    
    public function problemNumber()
    {
        return (string) $this->problemNumber; // workaround 1
    }
}

ChangeInspectionDetailsHandler.php

<?php

class ChangeInspectionDetailsHandler
{
    public function handle(ChangeInspectionDetailsCommand $command)
    {
        // ...
        $contract->changeInspectionDetails(
            #$command->problemNumber() // no effect
            (string) $command->problemNumber() // workaround 2
        );
    }
}

Contract.php

<?php

class Contract
{
    public $problemNumber = ''; // no effect
    
    public function changeInspectionDetails(
        string $problemNumber = ''
    ) {
        $this->problemNumber = $problemNumber;

    }
}

Related issue: webdevilopers/php-ddd#18