Improved null handling for dates within list view.

A null value in a datetime field was rendering as the current date and time; this commit changes that to render a non-breaking space instead.

Resources/views/CRUD/list_datetime.html.twig

@@ -11,5 +11,10 @@ file that was distributed with this source code.
 
 {% extends 'SonataAdminBundle:CRUD:base_list_field.html.twig' %}
 
-{% block field%}{{ value|date }}{% endblock %}
-
+{% block field %}
+    {%- if value is empty -%}
+         
+    {%- else -%}
+        {{ value|date }}
+    {%- endif -%}
+{% endblock %}