feat: Don't show group member count if no value for it

On the group list and user pages, we get the member count for each group and we can display it. On the dataset groups tab, we don't have the member count for each group, so we should not display anything rather than falsely saying each group has no members.

ckanext/switzerland/templates/group/snippets/group_item.html

@@ -1,7 +1,8 @@
 {% block members %}
   {% if 'member_count' in group and group.member_count %}
     <strong class="count">{{ ungettext('{num} Member', '{num} Members', group.member_count).format(num=group.member_count) }}</strong>
-  {% else %}
+  {% elif 'member_count' in group %}
     <span class="count">{{ _('0 Members') }}</span>
+  {% else %}
   {% endif %}
 {% endblock %}