author metadata: no corresponding warnings

merges #526

Author: jashapiro

content/00.front-matter.md

@@ -70,9 +70,9 @@ Published: {{manubot.date_long}}
 
 ::: {#correspondence}
 ✉ — Correspondence possible via {% if manubot.ci_source is defined -%}[GitHub Issues](https://github.com/{{manubot.ci_source.repo_slug}}/issues){% else %}GitHub Issues{% endif %}
-{% if manubot.authors|map(attribute='corresponding')|select|max -%}
+{% if manubot.authors|map(attribute='corresponding',default=false)|select|max -%}
 or email to
-{% for author in manubot.authors|selectattr("corresponding") -%}
+{% for author in manubot.authors|selectattr("corresponding", "defined")|selectattr("corresponding") -%}
 {{ author.name }} \<{{ author.email }}\>{{ ", " if not loop.last else "." }}
 {% endfor %}
 {% endif %}