[Admin][PaymentMethod] Add validation to sandbox paypal fields (#409)

| Q               | A
| --------------- | -----
| Branch?         | 1.7
| Bug fix?        | kinda
| New feature?    | no
| Related tickets | -

Author: GSadee

src/Form/Type/PayPalConfigurationType.php

@@ -19,6 +19,7 @@
 use Symfony\Component\Form\FormBuilderInterface;
 use Symfony\Component\Form\FormEvent;
 use Symfony\Component\Form\FormEvents;
+use Symfony\Component\Validator\Constraints\NotBlank;
 
 final class PayPalConfigurationType extends AbstractType
 {
@@ -48,9 +49,18 @@ public function buildForm(FormBuilderInterface $builder, array $options): void
                 $form
                     ->add('sylius_merchant_id', HiddenType::class, ['data' => self::SANDBOX_SYLIUS_MERCHANT_ID, 'attr' => ['readonly' => true]])
                     ->add('partner_attribution_id', HiddenType::class, ['data' => self::SANDBOX_ATTRIBUTION_ID, 'attr' => ['readonly' => true]])
-                    ->add('client_id', TextType::class, ['label' => 'sylius.pay_pal.client_id'])
-                    ->add('client_secret', TextType::class, ['label' => 'sylius.pay_pal.client_secret'])
-                    ->add('merchant_id', TextType::class, ['label' => 'sylius.pay_pal.merchant_id'])
+                    ->add('client_id', TextType::class, [
+                        'label' => 'sylius.pay_pal.client_id',
+                        'constraints' => [new NotBlank(['groups' => 'sylius'])],
+                    ])
+                    ->add('client_secret', TextType::class, [
+                        'label' => 'sylius.pay_pal.client_secret',
+                        'constraints' => [new NotBlank(['groups' => 'sylius'])],
+                    ])
+                    ->add('merchant_id', TextType::class, [
+                        'label' => 'sylius.pay_pal.merchant_id',
+                        'constraints' => [new NotBlank(['groups' => 'sylius'])],
+                    ])
                 ;
             } else {
                 $form