Fix changelog template

Author: ericof

backend/news/.changelog_template.jinja

@@ -1,9 +1,9 @@
-{% if sections[] %}
-{% for category, val in definitions.items() if category in sections[] %}
+{% if sections[""] %}
+{% for category, val in definitions.items() if category in sections[""] %}
 
 ### {{ definitions[category]['name'] }}
 
-{% for text, values in sections[][category].items() %}
+{% for text, values in sections[""][category].items() %}
 - {{ text }} {{ values|join(', ') }}
 {% endfor %}