$builder->add('contactPerson', EntityType::class, [
    'class' => 'AppBundle:User',
    'choice_label' => 'username',
    'empty_data' => null,
    'required' => false,
    // 'placeholder' => '' - Can be ommited, empty value is default
]);

manyToOne:
    contactPerson:
        targetEntity: AppBundleEntityUser
        joinColumn:
            name: contact_person
            referencedColumnName: id
            onDelete: RESTRICT
            nullable: true
        fetch: EAGER

public function editAction(Request $request, $id)
    {
        /** @var CountryService $countryService */
        $countryService = $this->get('app.country');
        /** @var Country $country */
        $country = $countryService->findById($id);

        if (!$country) {
            $this->addFlash('error', 'back.country.not_found');
            return $this->redirectToRoute('back_country_list');
        }

        $form = $this->createForm(
            CountryType::class,
            $country,
            [
                'edit_action' => true
            ]
        );
        $form->handleRequest($request);

        if ($form->isValid()) {
            $countryService->edit($country);

            $this->addFlash('success', 'back.country.edited');
            return $this->redirectToRoute('back_country_edit', ['id' => $id]);
        }

        return $this->render(
            'AppBundle:backcountry:edit.html.twig',
            [
                'form' => $form->createView()
            ]
        );
    }

"contactPerson" => ""