FIX: #152 add required false option to collections field in PageType (#153)

| Q               | A
| --------------- | -----
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Related tickets | fixes #152 
| License         | MIT

When creating a page the collections field is marked as required
currently. However, as there is no validation behind that, you can still
creating a page without select a collection. But as choosing a
collection is optional (same as when creating a block, or media) the
`collections` field in the `PageType` class should be set to `required =
false`.

Author: TheMilek

src/Form/Type/PageType.php

@@ -61,6 +61,7 @@ public function buildForm(FormBuilderInterface $builder, array $options): void
                 'label' => 'sylius_cms.ui.collections',
                 'multiple' => true,
                 'by_reference' => false,
+                'required' => false,
             ])
             ->add('channels', ChannelChoiceType::class, [
                 'label' => 'sylius_cms.ui.channels',