Check if collection has successor

Closes #42 (closed)

ckanext/odsh/templates/package/read.html

@@ -4,7 +4,7 @@
 {% set collection = h.get_collection(pkg) %}
 {% set collection_title = collection['title'] if collection else None %}
 {% set collection_url = collection['url'] if collection else None %}
-{% set successor_url = collection['successor']['url'] if collection else None %}
+{% set successor_url = collection['successor']['url'] if collection and 'successor' in collection else None %}
 {% set predecessor_url = collection['predecessor']['url'] if collection else None %}
 {% set latest_collection_member = collection['persistent_link_last_member'] if collection else None %}
 {% set stars = h.odsh_openness_score_dataset_html(pkg) %}