Fix form when we do not use any resource factory (#194)

When we do not use any resource factory (eg: on resource detached from
Doctrine), we have an error cause the resource is not defined.

src/BootstrapAdminUi/templates/shared/crud/common/content/form.html.twig

@@ -3,7 +3,7 @@
 {% endif %}
 
 {% if resource is not defined %}
-    {% set resource = hookable_metadata.context.resource %}
+    {% set resource = hookable_metadata.context.resource|default(null) %}
 {% endif %}
 
 {% form_theme form '@SyliusBootstrapAdminUi/shared/form_theme.html.twig' %}