fix: adjust changelog message template to handle empty descriptions (#32)

Author: bdraco

templates/CHANGELOG.md.j2

@@ -9,7 +9,9 @@
 ### {{ category | capitalize }}
 {# List actual changes in the category #}
 {%- for commit in commits %}
+{% if commit is not none and commit.descriptions is defined %}
 - {{ commit.descriptions[0] | capitalize }} ([`{{ commit.short_hash }}`]({{ commit.hexsha | commit_hash_url }}))
+{% endif %}
 {%- endfor %}{# for commit #}
 
 {%- endfor %}{# for category, commits #}