fix(release): 🐛 don't crash the changelog on an unresolved contributor

git-cliff's contributor template sorted github.contributors by
username unconditionally; a contributor whose commit email doesn't
resolve to a GitHub login gets username=null, and Tera's sort filter
errors out on a null value ("Filter call 'sort' failed"), which
crashed the v1.2.0 release job before the PyPI publish step could
even run. Drop the sort (cosmetic ordering only) and skip rendering
any contributor without a resolved username instead of crashing.

Author: Laurent Guerard

cliff.toml

@@ -11,8 +11,9 @@ body = """
 {% if github.contributors | length > 0 %}
 ### 👥 Contributors
 
-{% for contributor in github.contributors | sort(attribute="username") %}\
-* @{{ contributor.username }}
+{% for contributor in github.contributors %}\
+{% if contributor.username %}* @{{ contributor.username }}
+{% endif %}\
 {% endfor %}
 {% endif %}\
 """